Online browser + post detail: SPA talks to e621 directly

Backend:
- POST /api/files/lookup/ reports which MD5s are already in the library

Frontend:
- e621 client extended with post/tag/favorite types and helpers: post
  search, post detail, batch posts by id, tag autocomplete, toggle
  favorite
- /online: tag search with autocomplete, post grid with rating colors
  and in-library badges, numbered pagination, page tag cloud, blacklist
  panel and filtered counts from /users/me.json, anonymous hint
- /online/view/🆔 media viewer (sample or original, video support),
  taxonomy-colored tags by category, specs sheet, favorite/unfavorite,
  description, sources, pools and parent/children thumbnails
- Shared CollapsibleSidebar extracted from the library page; Online
  added to the nav, command palette and sidebar toggle

frontend/src/components/CommandPalette.tsx

@@ -1,5 +1,6 @@
 import {
   FolderOpen,
+  Globe,
   LogIn,
   LogOut,
   Menu,
@@ -56,6 +57,12 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
         icon: Upload,
         run: () => navigate("/upload"),
       },
+      {
+        id: "online",
+        label: "Go to Online browser",
+        icon: Globe,
+        run: () => navigate("/online"),
+      },
       {
         id: "filters",
         label: "Toggle filters",