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/lib/nav.ts

@@ -0,0 +1,14 @@
+import type { Page } from "./urlState";
+
+// Tiny decoupled navigator so deep components (a feed card's download toast, the Download
+// Center) can switch pages without threading setPage through the whole tree. App registers
+// its setPage on mount; navigateTo is a no-op until then.
+let _navigate: ((p: Page) => void) | null = null;
+
+export function setNavigator(fn: (p: Page) => void): void {
+  _navigate = fn;
+}
+
+export function navigateTo(p: Page): void {
+  _navigate?.(p);
+}