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/lib/components/AddRepositoryModal.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+	import FolderPicker from '$lib/components/FolderPicker.svelte';
+
 	let {
 		onClose,
 		onAdded
@@ -97,19 +99,23 @@
 		</div>
 
 		<div class="space-y-4">
-			<label class="block">
+			<div class="block">
 				<span class="text-sm font-medium text-gray-700">
-					{source === 'github' ? 'GitHub URL' : 'Absolute Path'}
+					{source === 'github' ? 'GitHub URL' : 'Local Path'}
 				</span>
-				<input
-					type="text"
-					bind:value={sourceUrl}
-					placeholder={source === 'github'
-						? 'https://github.com/facebook/react'
-						: '/home/user/projects/my-sdk'}
-					class="mt-1 w-full rounded-lg border border-gray-300 px-3 py-2 text-sm focus:border-blue-500 focus:ring-1 focus:ring-blue-500 focus:outline-none"
-				/>
-			</label>
+				{#if source === 'github'}
+					<input
+						type="text"
+						bind:value={sourceUrl}
+						placeholder="https://github.com/facebook/react"
+						class="mt-1 w-full rounded-lg border border-gray-300 px-3 py-2 text-sm focus:border-blue-500 focus:ring-1 focus:ring-blue-500 focus:outline-none"
+					/>
+				{:else}
+					<div class="mt-1">
+						<FolderPicker bind:value={sourceUrl} onselect={(p) => { if (!title) title = p.split('/').at(-1) ?? ''; }} />
+					</div>
+				{/if}
+			</div>
 
 			<label class="block">
 				<span class="text-sm font-medium text-gray-700">Display Title (optional)</span>