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/AppShell.svelte

@@ -2,6 +2,7 @@
 	import { page } from '$app/state';
 	import {
 		BookOpen,
+		Brain,
 		CalendarDays,
 		CircleDot,
 		Compass,
@@ -39,6 +40,7 @@
 		{ id: 'inventory', href: '/inventory', label: 'Inventory', icon: Package2 },
 		{ id: 'reader', href: '/reader', label: 'Reader', icon: BookOpen },
 		{ id: 'media', href: '/media', label: 'Media', icon: LibraryBig },
+		{ id: 'brain', href: '/brain', label: 'Brain', icon: Brain },
 		{ id: 'settings', href: '/settings', label: 'Settings', icon: Settings2 }
 	];
 

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}>