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/CollapsibleSidebar.tsx

@@ -0,0 +1,47 @@
+import type { ReactNode } from "react";
+import { X } from "lucide-react";
+
+import { cn } from "@/lib/cn";
+import { useUi } from "@/store/ui";
+
+export function CollapsibleSidebar({
+  title = "Filters",
+  children,
+}: {
+  title?: string;
+  children: ReactNode;
+}) {
+  const sidebarOpen = useUi((state) => state.sidebarOpen);
+  const setSidebarOpen = useUi((state) => state.setSidebarOpen);
+
+  if (!sidebarOpen) return null;
+
+  return (
+    <>
+      <div
+        className="fixed inset-0 z-40 bg-ctp-crust/60 backdrop-blur-sm lg:hidden"
+        onClick={() => setSidebarOpen(false)}
+        aria-hidden="true"
+      />
+      <aside
+        className={cn(
+          "fixed inset-y-0 left-0 z-50 flex w-72 flex-col gap-5 overflow-y-auto border-r border-ctp-surface0 bg-ctp-base p-4",
+          "lg:sticky lg:inset-y-auto lg:top-20 lg:z-auto lg:w-64 lg:shrink-0 lg:overflow-visible lg:rounded-lg lg:border",
+        )}
+      >
+        <div className="flex items-center justify-between">
+          <span className="text-sm font-semibold text-ctp-subtext1">{title}</span>
+          <button
+            type="button"
+            onClick={() => setSidebarOpen(false)}
+            title="Close filters"
+            className="rounded-md p-1 text-ctp-overlay0 transition hover:bg-ctp-surface0 hover:text-ctp-text lg:hidden"
+          >
+            <X className="h-4 w-4" />
+          </button>
+        </div>
+        {children}
+      </aside>
+    </>
+  );
+}