fix(ui): resolve state_referenced_locally warnings and add folder picker

- Fix Svelte state_referenced_locally warning in +page.svelte and repos/[id]/+page.svelte
  by initializing $state with empty defaults and syncing via $effect
- Add FolderPicker component with server-side filesystem browser
  (single-click to navigate, double-click or "Select This Folder" to confirm)
- Git repos highlighted with orange folder icon and "git" badge
- Add GET /api/v1/fs/browse endpoint listing subdirectories
- Wire FolderPicker into AddRepositoryModal for local source type
- Auto-fills title from the selected folder name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/repos/[id]/+page.svelte

@@ -7,9 +7,13 @@
 
 	let { data }: { data: PageData } = $props();
 
-	// Local mutable copies updated via fetch — intentionally capturing initial server values.
-	let repo = $state<Repository & { versions?: RepositoryVersion[] }>(data.repo); // svelte-disable state_referenced_locally
-	let recentJobs = $state<IndexingJob[]>(data.recentJobs ?? []); // svelte-disable state_referenced_locally
+	// Initialized empty; $effect syncs from data prop on every navigation/reload.
+	let repo = $state<Repository & { versions?: RepositoryVersion[] }>({} as Repository & { versions?: RepositoryVersion[] });
+	let recentJobs = $state<IndexingJob[]>([]);
+	$effect(() => {
+		if (data.repo) repo = data.repo;
+		recentJobs = data.recentJobs ?? [];
+	});
 	let showDeleteConfirm = $state(false);
 	let activeJobId = $state<string | null>(null);
 	let errorMessage = $state<string | null>(null);