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/api/v1/fs/browse/+server.ts

@@ -0,0 +1,36 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import fs from 'node:fs';
+import path from 'node:path';
+import os from 'node:os';
+
+export const GET: RequestHandler = ({ url }) => {
+	const rawPath = url.searchParams.get('path') ?? os.homedir();
+	const target = path.resolve(rawPath);
+
+	let entries: { name: string; path: string; isGitRepo: boolean }[] = [];
+	let error: string | null = null;
+	let resolved = target;
+
+	try {
+		const items = fs.readdirSync(target, { withFileTypes: true });
+		entries = items
+			.filter((d) => d.isDirectory() && !d.name.startsWith('.'))
+			.map((d) => {
+				const full = path.join(target, d.name);
+				const isGitRepo = fs.existsSync(path.join(full, '.git'));
+				return { name: d.name, path: full, isGitRepo };
+			})
+			.sort((a, b) => {
+				// git repos first, then alphabetical
+				if (a.isGitRepo !== b.isGitRepo) return a.isGitRepo ? -1 : 1;
+				return a.name.localeCompare(b.name);
+			});
+	} catch (e) {
+		error = (e as NodeJS.ErrnoException).code === 'EACCES' ? 'Permission denied' : 'Path not found';
+	}
+
+	const parent = target !== path.parse(target).root ? path.dirname(target) : null;
+
+	return json({ path: resolved, parent, entries, error });
+};