Fix desktop showing the mobile layout: decide layout from the actual build platform (cfg!(mobile)) instead of sniffing the webview user-agent, which some WebKitGTK builds report mobile-looking (#63)

src/lib/stores/app.ts

@@ -85,6 +85,11 @@ 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),
+);
 export const androidApkUrl = writable<string | null>(null);
 
 // Install types that handle their own updates (in-app auto-updater, or a