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/VideoCard.tsx

@@ -11,6 +11,7 @@ import {
 } from "lucide-react";
 import Avatar from "./Avatar";
 import AddToPlaylist from "./AddToPlaylist";
+import DownloadButton from "./DownloadButton";
 import clsx from "clsx";
 import type { Video } from "../lib/api";
 import { formatDuration, formatViews, relativeTime } from "../lib/format";
@@ -67,6 +68,7 @@ function Actions({
         <Bookmark className="w-4 h-4" />
       </button>
       <AddToPlaylist videoId={video.id} />
+      <DownloadButton videoId={video.id} title={video.title} />
       <button
         onClick={act("hidden")}
         title={video.status === "hidden" ? t("card.unhide") : t("card.hide")}