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/components/NavSidebar.tsx

@@ -8,6 +8,7 @@ import {
   Bell,
   ChevronLeft,
   ChevronRight,
+  Download,
   Home,
   Info,
   ListVideo,
@@ -147,6 +148,13 @@ export default function NavSidebar({
     { intervalMs: 30000, enabled: !me.is_demo }
   );
   const msgUnread = msgUnreadQuery.data?.count ?? 0;
+  // Download center badge = items still working (queued/running/paused). Reuses the same
+  // lightweight per-user index the feed cards poll, so no extra request shape.
+  const dlIndexQuery = useLiveQuery(["download-index"], api.downloadIndex, {
+    intervalMs: 30000,
+    enabled: !me.is_demo,
+  });
+  const dlActive = Object.values(dlIndexQuery.data ?? {}).filter((s) => s !== "done").length;
 
   type NavItem = { page: Page; icon: typeof Home; label: string; badge?: number };
   // User-facing content modules vs. admin/system modules, separated by a divider in the rail.
@@ -159,6 +167,11 @@ export default function NavSidebar({
     ...(me.is_demo
       ? []
       : [{ page: "messages" as Page, icon: MessageSquare, label: t("messages.navLabel"), badge: msgUnread }]),
+    // Download center — YouTube → server → your device. Hidden for the shared demo account
+    // (spends server disk + needs a real identity).
+    ...(me.is_demo
+      ? []
+      : [{ page: "downloads" as Page, icon: Download, label: t("downloads.navLabel"), badge: dlActive }]),
     // Per-user sync status + your own API usage (admins get an extra system-wide tab inside).
     { page: "stats", icon: BarChart3, label: t("header.account.stats") },
   ];