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/AppShell.tsx

@@ -10,10 +10,12 @@ import { api } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import type { SystemStatus } from "@/lib/types";
 import { useAuth } from "@/store/auth";
+import { useE621 } from "@/store/e621";
 import { useUi } from "@/store/ui";
 
 const navItems = [
   { to: "/", label: "Library" },
+  { to: "/online", label: "Online" },
   { to: "/upload", label: "Upload" },
 ];
 
@@ -21,9 +23,14 @@ export function AppShell() {
   const user = useAuth((state) => state.user);
   const logout = useAuth((state) => state.logout);
   const toggleSidebar = useUi((state) => state.toggleSidebar);
+  const loadE621 = useE621((state) => state.load);
   const location = useLocation();
   const [paletteOpen, setPaletteOpen] = useState(false);
 
+  useEffect(() => {
+    void loadE621();
+  }, [loadE621]);
+
   const statusQuery = useQuery({
     queryKey: ["status"],
     queryFn: () => api<SystemStatus>("/api/status/"),
@@ -53,7 +60,8 @@ export function AppShell() {
     return () => window.removeEventListener("keydown", handleKeyDown);
   }, [toggleSidebar]);
 
-  const showSidebarToggle = location.pathname === "/";
+  const showSidebarToggle =
+    location.pathname === "/" || location.pathname === "/online";
 
   return (
     <div className="flex min-h-screen flex-col">