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/PlayerModal.tsx

@@ -5,6 +5,7 @@ import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { AlertTriangle, ArrowLeft, Check, CheckCheck, ExternalLink, SkipBack, SkipForward, Volume2, VolumeX, X } from "lucide-react";
 import Avatar from "./Avatar";
 import AddToPlaylist from "./AddToPlaylist";
+import DownloadButton from "./DownloadButton";
 import { api, type Video } from "../lib/api";
 import { channelYouTubeUrl, formatDuration, formatViews, relativeTime } from "../lib/format";
 import { renderDescription } from "../lib/descriptionLinks";
@@ -630,6 +631,13 @@ export default function PlayerModal({
                 className="ml-auto shrink-0 inline-flex items-center justify-center w-9 h-9 rounded-lg text-muted hover:text-fg hover:bg-surface border border-border transition"
               />
             )}
+            {!navigated && (
+              <DownloadButton
+                videoId={active.id}
+                title={active.title}
+                className="shrink-0 inline-flex items-center justify-center w-9 h-9 rounded-lg text-muted hover:text-fg hover:bg-surface border border-border transition"
+              />
+            )}
             {!navigated && (
               <button
                 onClick={() => setWatched(!watched)}