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/routes/(app)/+layout.server.ts

@@ -24,7 +24,7 @@ export const load: LayoutServerLoad = async ({ cookies, url }) => {
 				// Hides nav items but does NOT block direct URL access.
 				// This is intentional: all shared services are accessible to all authenticated users.
 				// Hiding reduces clutter for users who don't need certain apps day-to-day.
-				const allApps = ['tasks', 'trips', 'fitness', 'inventory', 'budget', 'reader', 'media'];
+				const allApps = ['tasks', 'trips', 'fitness', 'inventory', 'budget', 'reader', 'media', 'brain'];
 				const hiddenByUser: Record<string, string[]> = {
 					'madiha': ['inventory', 'reader'],
 				};

frontend-v2/src/routes/(app)/brain/+page.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+  import { page } from '$app/state';
+  import AtelierBrainPage from '$lib/pages/brain/AtelierBrainPage.svelte';
+  const useAtelierShell = $derived((page as any).data?.useAtelierShell || false);
+</script>
+
+{#if useAtelierShell}
+  <AtelierBrainPage />
+{:else}
+  <AtelierBrainPage />
+{/if}