feat: brain sub-sidebar in AppShell — folders/tags under Brain nav item

- Brain nav item expands when on /brain page
- Shows Folders/Tags toggle tabs
- Folder links: /brain?folder=Work etc
- Tag links: /brain?tag=dev etc
- Counts shown next to each
- Brain page reads filter from URL params
- Only shows folders with items + key defaults (Home, Work, Knowledge)
- Tags limited to 12 most-used

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

frontend-v2/src/lib/pages/brain/AtelierBrainPage.svelte

@@ -307,6 +307,16 @@
   });
 
   onMount(async () => {
+    // Read filters from URL params (set by AppShell sidebar links)
+    const urlFolder = new URL(window.location.href).searchParams.get('folder');
+    const urlTag = new URL(window.location.href).searchParams.get('tag');
+    if (urlFolder) {
+      activeFolder = urlFolder;
+    }
+    if (urlTag) {
+      activeTag = urlTag;
+    }
+
     await loadSidebar();
     await loadItems();
   });