Notebooks: add a collapse-all / expand-all button to the sidebar tree header (#26)

src/lib/components/Sidebar.svelte

@@ -67,6 +67,22 @@
 			$collapsedNotebooks = [...$collapsedNotebooks, path];
 		}
 	}
+	// Collapse-all / expand-all: toggles every notebook that has children.
+	function collectParentPaths(tree: NotebookEntry[]): string[] {
+		const out: string[] = [];
+		for (const nb of tree) {
+			if (nb.children && nb.children.length > 0) {
+				out.push(nb.path);
+				out.push(...collectParentPaths(nb.children));
+			}
+		}
+		return out;
+	}
+	const allParentPaths = $derived(collectParentPaths($notebooks));
+	const allCollapsed = $derived(allParentPaths.length > 0 && allParentPaths.every((p) => $collapsedNotebooks.includes(p)));
+	function toggleAllCollapse() {
+		$collapsedNotebooks = allCollapsed ? [] : [...allParentPaths];
+	}
 
 
 	export async function refresh() {
@@ -578,6 +594,18 @@
 				}}
 			>
 				<span class="section-title">Notebooks</span>
+				<div class="section-actions">
+					{#if allParentPaths.length > 0}
+						<button class="icon-btn-sm" onclick={(e) => { e.stopPropagation(); toggleAllCollapse(); }} title={allCollapsed ? 'Expand all notebooks' : 'Collapse all notebooks'} aria-label={allCollapsed ? 'Expand all notebooks' : 'Collapse all notebooks'}>
+							<svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+								{#if allCollapsed}
+									<path d="m7 15 5 5 5-5" /><path d="m7 9 5-5 5 5" />
+								{:else}
+									<path d="m7 20 5-5 5 5" /><path d="m7 4 5 5 5-5" />
+								{/if}
+							</svg>
+						</button>
+					{/if}
 					<button class="icon-btn-sm" onclick={() => {
 						if (showNewNotebook) {
 							showNewNotebook = false; newNotebookName = ''; newNotebookParent = null;
@@ -592,6 +620,7 @@
 						</svg>
 					</button>
 				</div>
+			</div>
 
 			{#if showNewNotebook}
 				<div class="new-notebook-input">
@@ -952,6 +981,11 @@
 		color: var(--text-tertiary);
 	}
 
+	.section-actions {
+		display: flex;
+		align-items: center;
+		gap: 2px;
+	}
 	.icon-btn, .icon-btn-sm {
 		background: none;
 		border: none;