diff --git a/src-tauri/src/lib.rs b/src-tauri/src/lib.rs
index 364fe54..63ca13b 100644
--- a/src-tauri/src/lib.rs
+++ b/src-tauri/src/lib.rs
@@ -38,7 +38,21 @@ pub fn run() {
     let close_to_tray = config.close_to_tray && show_tray;
     let app_state = AppState::new(config);
 
+    // Inject the compile-time platform so the frontend never sniffs the (sometimes
+    // mobile-looking) WebKitGTK user-agent. (#63)
+    let platform_init = format!(
+        "window.__HELIX_PLATFORM__={{mobile:{},android:{},ios:{}}};",
+        cfg!(mobile),
+        cfg!(target_os = "android"),
+        cfg!(target_os = "ios"),
+    );
+
     let mut builder = tauri::Builder::default()
+        .plugin(
+            tauri::plugin::Builder::<tauri::Wry>::new("helix-platform")
+                .js_init_script(platform_init)
+                .build(),
+        )
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_fs::init())
         .plugin(tauri_plugin_opener::init())
diff --git a/src/lib/components/AppLayout.svelte b/src/lib/components/AppLayout.svelte
index 6035d1c..8a92e4e 100644
--- a/src/lib/components/AppLayout.svelte
+++ b/src/lib/components/AppLayout.svelte
@@ -58,9 +58,8 @@
 	const appWindow = getCurrentWindow();
 	const isMac = navigator.platform.startsWith('Mac');
 	const isMobile = $derived($platformIsMobile);
-	const isAndroid = /android/i.test(navigator.userAgent);
 	import { loadVaultState, saveVaultState, readNote, createDailyNote, createBackup, getPendingOpenFile, addQuickAccess, removeQuickAccess, getQuickAccess, setTheme, syncNow, setTaskDone, setTaskPriority, setTaskDue } from '$lib/api';
-	import { darkThemes } from '$lib/platform';
+	import { darkThemes, isAndroid } from '$lib/platform';
 	import { debounce } from '$lib/utils/debounce';
 	import { openNoteWindow } from '$lib/utils/window';
 	import { get } from 'svelte/store';
diff --git a/src/lib/components/Editor.svelte b/src/lib/components/Editor.svelte
index 0407aa1..432c85f 100644
--- a/src/lib/components/Editor.svelte
+++ b/src/lib/components/Editor.svelte
@@ -49,9 +49,9 @@
 	import { wrapTextareaSelection } from '$lib/editor/source/selectionPairs';
 	import GraphView from './GraphView.svelte';
 	import TagSuggestInput from './TagSuggestInput.svelte';
+	import { isMobile, isAndroid } from '$lib/platform';
 
 	const modKey = navigator.platform.startsWith('Mac') ? '⌘' : 'Ctrl';
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
 
 	// Track virtual keyboard height on mobile via visualViewport
 	let keyboardHeight = $state(0);
@@ -1028,7 +1028,6 @@
 			function addToolbar(container: HTMLElement, source: string) {
 				const toolbar = document.createElement('div');
 				toolbar.className = 'mermaid-render-toolbar';
-				const isAndroid = /android/i.test(navigator.userAgent);
 
 				if (!isAndroid) {
 					const copyBtn = document.createElement('button');
diff --git a/src/lib/components/GraphView.svelte b/src/lib/components/GraphView.svelte
index 9a0a44f..5191011 100644
--- a/src/lib/components/GraphView.svelte
+++ b/src/lib/components/GraphView.svelte
@@ -2,14 +2,13 @@
 	import { onDestroy } from 'svelte';
 	import { getGraphData } from '$lib/api';
 	import { activeNotePath, appConfig } from '$lib/stores/app';
+	import { isMobile } from '$lib/platform';
 
 	let { onclose, onnavigate }: {
 		onclose: () => void;
 		onnavigate: (path: string, title: string) => void;
 	} = $props();
 
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
-
 	let canvas = $state<HTMLCanvasElement>(null!);
 	let loading = $state(true);
 	let searchQuery = $state('');
diff --git a/src/lib/components/InfoPanel.svelte b/src/lib/components/InfoPanel.svelte
index 0df984d..6adf445 100644
--- a/src/lib/components/InfoPanel.svelte
+++ b/src/lib/components/InfoPanel.svelte
@@ -15,9 +15,9 @@
 		bindingsEqual,
 		type ActionDef,
 	} from '$lib/keybindings';
+	import { isMobile } from '$lib/platform';
 
 	const modKey = navigator.platform.startsWith('Mac') ? '⌘' : 'Ctrl';
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
 
 	// Customizable shortcuts, grouped for display.
 	const actionGroups: { title: string; actions: ActionDef[] }[] = [
diff --git a/src/lib/components/NoteList.svelte b/src/lib/components/NoteList.svelte
index 22385d4..e2c8b83 100644
--- a/src/lib/components/NoteList.svelte
+++ b/src/lib/components/NoteList.svelte
@@ -43,6 +43,7 @@
 	import type { NoteEntry, TrashNotebookEntry, SortMode, TaskItem } from '$lib/types';
 	import TasksView from './TasksView.svelte';
 	import TagSuggestInput from './TagSuggestInput.svelte';
+	import { isMobile, isAndroid } from '$lib/platform';
 
 	let { onNoteSelected = (_path: string, _content: string) => {}, onNoteMoved = () => {}, onBeforeNoteSwitch = () => {}, onNoteCreated = () => {}, onToggleTask = async (_t: TaskItem) => {}, onSetTaskPriority = async (_t: TaskItem, _p: string | null) => {}, onSetTaskDue = async (_t: TaskItem, _d: string | null) => {} }: {
 		onNoteSelected?: (path: string, content: string) => void;
@@ -55,8 +56,6 @@
 	} = $props();
 
 	const modKey = navigator.platform.startsWith('Mac') ? '⌘' : 'Ctrl';
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
-	const isAndroid = /android/i.test(navigator.userAgent);
 	let multiSelectMode = $state(false);
 	let trashNotebooks = $state<TrashNotebookEntry[]>([]);
 	let trashBusy = $state<string | null>(null);
diff --git a/src/lib/components/SearchPanel.svelte b/src/lib/components/SearchPanel.svelte
index 0e00f77..1909a61 100644
--- a/src/lib/components/SearchPanel.svelte
+++ b/src/lib/components/SearchPanel.svelte
@@ -3,6 +3,7 @@
 	import { searchNotes, readNote } from '$lib/api';
 	import { debounce } from '$lib/utils/debounce';
 	import type { SearchResult } from '$lib/types';
+	import { isMobile } from '$lib/platform';
 
 	let query = $state('');
 	let results = $state<SearchResult[]>([]);
@@ -83,8 +84,6 @@
 		return str.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
 	}
 
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
-
 	async function openResult(result: SearchResult) {
 		try {
 			const content = await readNote(result.path);
diff --git a/src/lib/components/SettingsPanel.svelte b/src/lib/components/SettingsPanel.svelte
index 31ba7a0..c5caf31 100644
--- a/src/lib/components/SettingsPanel.svelte
+++ b/src/lib/components/SettingsPanel.svelte
@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { showSettings, theme, appConfig, updateAvailable as globalUpdateAvailable, updateObj as globalUpdateObj, installType, settingsTab, vaultReady, androidApkUrl, checkForUpdateMobile, notebookSortMode, isManagedInstall } from '$lib/stores/app';
 	import { setTheme, setAccentColor, setFontSize, setFontFamily, setLineHeight, setUiScale, setContentWidth, setGeneralSettings, importObsidian, createBackup, listBackups, restoreBackup, deleteBackup, setBackupSettings, setAiSettings, testAiConnection, setSyncSettings, testSyncConnection, syncNow } from '$lib/api';
-	import { darkThemes } from '$lib/platform';
+	import { darkThemes, isMobile } from '$lib/platform';
 	import { open as openDialog } from '@tauri-apps/plugin-dialog';
 	import { listen } from '@tauri-apps/api/event';
 	import { getVersion } from '@tauri-apps/api/app';
@@ -9,7 +9,6 @@
 	import { openUrl } from '$lib/api';
 	import type { ImportResult, BackupEntry } from '$lib/types';
 
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
 	const modKey = navigator.platform.startsWith('Mac') ? '⌘' : 'Ctrl';
 
 	type Tab = 'general' | 'editor' | 'styling' | 'import' | 'backup' | 'ai' | 'sync' | 'updates';
diff --git a/src/lib/components/Sidebar.svelte b/src/lib/components/Sidebar.svelte
index 2c30af7..22019e5 100644
--- a/src/lib/components/Sidebar.svelte
+++ b/src/lib/components/Sidebar.svelte
@@ -26,13 +26,13 @@
 	import { readFile } from '@tauri-apps/plugin-fs';
 	import { convertFileSrc } from '@tauri-apps/api/core';
 	import type { NotebookEntry } from '$lib/types';
+	import { isMobile } from '$lib/platform';
 
 	let { onViewChanged = () => {} }: {
 		onViewChanged?: () => void;
 	} = $props();
 
 	const modKey = navigator.platform.startsWith('Mac') ? '⌘' : 'Ctrl';
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
 
 	// Whether any top nav item (All Notes, Quick Access, Tasks, Daily Notes, Trash) is visible.
 	// When all are hidden we drop the empty <nav> and its divider so the tree sits flush.
diff --git a/src/lib/platform.ts b/src/lib/platform.ts
index 6a9d96a..ded4fc7 100644
--- a/src/lib/platform.ts
+++ b/src/lib/platform.ts
@@ -1,10 +1,19 @@
-// Centralized platform detection. iOS WebViews report iPhone/iPad/iPod in the user
-// agent (not the literal "ios"), so they must be matched explicitly.
+// Platform is the compile-time build target, injected by the backend into
+// window.__HELIX_PLATFORM__ before app scripts run (see src-tauri/src/lib.rs). The UA is only a
+// fallback when that global is absent (SSR/prerender, plain-browser dev): some desktop WebKitGTK
+// builds report a mobile-looking user-agent. (#63)
+type HelixPlatform = { mobile: boolean; android: boolean; ios: boolean };
+
+const injected: HelixPlatform | undefined =
+	typeof window !== 'undefined'
+		? (window as unknown as { __HELIX_PLATFORM__?: HelixPlatform }).__HELIX_PLATFORM__
+		: undefined;
+
 const ua = typeof navigator !== 'undefined' ? navigator.userAgent : '';
 
-export const isAndroid = /android/i.test(ua);
-export const isIOS = /iphone|ipad|ipod/i.test(ua);
-export const isMobile = isAndroid || isIOS;
+export const isAndroid = injected ? injected.android : /android/i.test(ua);
+export const isIOS = injected ? injected.ios : /iphone|ipad|ipod/i.test(ua);
+export const isMobile = injected ? injected.mobile : isAndroid || isIOS;
 
 // Themes that use the dark color scheme. Used by applyTheme() to toggle the
 // `dark` class on the root element. Keep this in sync when adding new themes.
diff --git a/src/lib/stores/app.ts b/src/lib/stores/app.ts
index 13ae8da..931e930 100644
--- a/src/lib/stores/app.ts
+++ b/src/lib/stores/app.ts
@@ -1,4 +1,5 @@
 import { writable, derived, get } from "svelte/store";
+import { isMobile } from "$lib/platform";
 import type {
   AppConfig,
   NoteEntry,
@@ -87,11 +88,8 @@ export const updateAvailable = writable<{
 } | null>(null);
 export const updateObj = writable<any>(null);
 export const installType = writable<string>("native");
-// True only on the Android/iOS build. Defaults to a user-agent guess for the first paint, then
-// gets the authoritative compile-time value from the backend at startup (see +layout). (#63)
-export const platformIsMobile = writable<boolean>(
-  typeof navigator !== "undefined" && /android|iphone|ipad|ipod/i.test(navigator.userAgent),
-);
+// True only on the Android/iOS build. Reconfirmed from the backend at startup (see +layout). (#63)
+export const platformIsMobile = writable<boolean>(isMobile);
 export const androidApkUrl = writable<string | null>(null);
 
 // Install types that handle their own updates (in-app auto-updater, or a
diff --git a/src/routes/+layout.svelte b/src/routes/+layout.svelte
index a5058bf..e6a839a 100644
--- a/src/routes/+layout.svelte
+++ b/src/routes/+layout.svelte
@@ -4,13 +4,11 @@
 	import { theme, appConfig, activeNote, activeNotePath, installType, platformIsMobile, checkForUpdate, checkForUpdateMobile, isManagedInstall } from '$lib/stores/app';
 	import { openFile, openUrl, readNote, getInstallType, isMobilePlatform } from '$lib/api';
 	import { get } from 'svelte/store';
-	import { darkThemes } from '$lib/platform';
+	import { darkThemes, isMobile, isAndroid } from '$lib/platform';
 	import ResizeHandles from '$lib/components/ResizeHandles.svelte';
 
 	let { children } = $props();
 
-	const isMobile = /android|iphone|ipad|ipod/i.test(navigator.userAgent);
-
 	// Reactively apply theme class to <html> whenever $theme changes
 	$effect(() => {
 		applyTheme($theme);
@@ -46,8 +44,6 @@
 		return resolved.join('/');
 	}
 
-	const isAndroid = /android/i.test(navigator.userAgent);
-
 	function openLocalFile(path: string) {
 		if (isAndroid) {
 			const bridge = (window as any).Android;