feat(downloads): M5 — Download Center frontend

- api.ts: download types + methods (profiles, enqueue, list/manage, file url, share,
  usage, index, admin storage/quota)
- format.ts: formatBytes / formatSpeed
- i18n: en/hu/de downloads.json (trilingual)
- Downloads nav module (Download icon, active-count badge, hidden for demo) placed after
  Messages; urlState page + App render + Header title
- DownloadButton: self-contained per-card/player affordance (demo-hidden, reflects the
  per-user download index: downloaded / in-queue), opens the preset dialog
- DownloadDialog: preset picker + optional display name -> enqueue + 'View' toast
- ProfileEditor: manage built-in + custom formats (full spec form incl. SponsorBlock)
- DownloadCenter: Queue / Library / Shared / (admin) System tabs, live progress via
  useLiveQuery, usage bar, pause/resume/cancel/delete, save-to-device, rename, share,
  admin storage dashboard + per-user quota editor
- wired DownloadButton into VideoCard + PlayerModal
- lib/nav.ts: decoupled navigator so the download toast can jump to the page

tsc + vite build clean. Verified in a headless authed browser: page + tabs render, Library
shows a real download with usage bar + actions, no console errors.

frontend/src/App.tsx

@@ -57,6 +57,8 @@ import AdminUsers, { focusAccessRequestsTab } from "./components/AdminUsers";
 import SettingsPanel from "./components/SettingsPanel";
 import NotificationsPanel from "./components/NotificationsPanel";
 import Messages from "./components/Messages";
+import DownloadCenter from "./components/DownloadCenter";
+import { setNavigator } from "./lib/nav";
 import ChatDock from "./components/ChatDock";
 import OnboardingWizard from "./components/OnboardingWizard";
 import { shouldAutoOpenOnboarding } from "./lib/onboarding";
@@ -279,6 +281,8 @@ export default function App() {
     }
     go();
   }
+  // Expose the current setPage to decoupled callers (download toast "View", etc.).
+  useEffect(() => setNavigator(setPage));
 
   function setSidebarLayout(next: SidebarLayout) {
     setSidebarLayoutState(next);
@@ -728,6 +732,8 @@ export default function App() {
             <div className="p-4 max-w-3xl w-full mx-auto">
               <Messages meId={meQuery.data!.id} />
             </div>
+          ) : page === "downloads" && !meQuery.data!.is_demo ? (
+            <DownloadCenter me={meQuery.data!} />
           ) : page === "settings" ? (
             <SettingsPanel
               me={meQuery.data!}