feat: add search button to sidebar with Ctrl+K shortcut hint

Compact search bar button between sidebar header and nav. Shows
magnifying glass icon + placeholder text + ⌘K kbd hint when expanded,
just the icon when collapsed. Clicking opens the search palette.

frontend/src/lib/components/SearchPalette.svelte

@@ -17,12 +17,17 @@
 		commandTrackersCache,
 	} from '$lib/stores/caches.svelte';
 
+	let { onopen = undefined }: { onopen?: (opener: () => void) => void } = $props();
+
 	let open = $state(false);
 	let query = $state('');
 	let activeIndex = $state(0);
 	let loading = $state(false);
 	let inputEl: HTMLInputElement;
 
+	// Expose openPalette to parent via callback
+	$effect(() => { onopen?.(openPalette); });
+
 	interface SearchResult {
 		id: number;
 		name: string;