feat: brain frontend — Atelier-style page with capture, search, feed, detail sheet

- AtelierBrainPage.svelte with full CRUD UI
- Capture bar: paste URL or type note, saves instantly
- Folder signal cards with counts
- Hybrid search (keyword + semantic)
- Item feed with metadata, tags, confidence indicators
- Detail slide-over sheet with summary, metadata, actions
- Added to AppShell nav, legacy Navbar, and layout allApps
- Route at /brain

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend-v2/src/lib/components/layout/Navbar.svelte

@@ -63,6 +63,9 @@
 			{#if showApp('media')}
 				<a href="/media" class="navbar-link" class:active={isActive('/media')}>Media</a>
 			{/if}
+			{#if showApp('brain')}
+				<a href="/brain" class="navbar-link" class:active={isActive('/brain')}>Brain</a>
+			{/if}
 		</div>
 
 		<button class="search-trigger" onclick={onOpenCommand}>