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/i18n/locales/en/downloads.json

@@ -0,0 +1,136 @@
+{
+  "navLabel": "Downloads",
+  "button": {
+    "label": "Download",
+    "queued": "In queue",
+    "downloaded": "Downloaded"
+  },
+  "dialog": {
+    "title": "Download video",
+    "profile": "Format",
+    "nameLabel": "File name (optional)",
+    "namePlaceholder": "Leave blank to use the video title",
+    "submit": "Add to downloads",
+    "added": "Added to downloads",
+    "view": "View",
+    "manage": "Manage formats"
+  },
+  "page": {
+    "title": "Downloads",
+    "subtitle": "Download videos to the server, then save them to your device.",
+    "addUrl": "Paste a YouTube link or video id…",
+    "add": "Add",
+    "manage": "Manage formats"
+  },
+  "tabs": {
+    "queue": "Queue",
+    "done": "Library",
+    "shared": "Shared with me",
+    "system": "System"
+  },
+  "status": {
+    "queued": "Queued",
+    "running": "Downloading",
+    "paused": "Paused",
+    "done": "Ready",
+    "error": "Failed",
+    "canceled": "Canceled",
+    "expired": "Expired"
+  },
+  "actions": {
+    "pause": "Pause",
+    "resume": "Resume",
+    "cancel": "Cancel",
+    "delete": "Remove",
+    "retry": "Retry",
+    "download": "Save to my device",
+    "rename": "Rename",
+    "share": "Share"
+  },
+  "empty": {
+    "queue": "Nothing in the queue.",
+    "done": "No downloads yet.",
+    "shared": "Nothing has been shared with you.",
+    "admin": "No downloads on this instance yet."
+  },
+  "usage": {
+    "title": "Your storage",
+    "items": "{{used}} / {{max}} items",
+    "unlimited": "Unlimited"
+  },
+  "rename": {
+    "title": "Rename download",
+    "label": "Display name",
+    "hint": "Only changes the name you see and download with — the stored file keeps its own name.",
+    "save": "Save"
+  },
+  "share": {
+    "title": "Share download",
+    "label": "Recipient email",
+    "placeholder": "user@example.com",
+    "submit": "Share",
+    "done": "Shared with {{email}}"
+  },
+  "confirm": {
+    "cancelTitle": "Cancel download?",
+    "cancelBody": "This stops the download and removes it from your queue.",
+    "deleteTitle": "Remove download?",
+    "deleteBody": "This removes it from your list. The file may stay cached for other users until it expires."
+  },
+  "profiles": {
+    "manage": "Manage formats",
+    "title": "Download formats",
+    "builtin": "Built-in",
+    "yours": "Your formats",
+    "new": "New format",
+    "name": "Name",
+    "namePlaceholder": "My format",
+    "mode": "Content",
+    "modeAv": "Video + audio",
+    "modeV": "Video only",
+    "modeA": "Audio only",
+    "quality": "Max quality",
+    "best": "Best",
+    "container": "Container",
+    "audioFormat": "Audio format",
+    "vcodec": "Video codec",
+    "any": "Any",
+    "embedSubs": "Embed subtitles",
+    "embedChapters": "Embed chapters",
+    "embedThumbnail": "Embed thumbnail",
+    "sponsorblock": "Skip sponsor segments (SponsorBlock)",
+    "save": "Save",
+    "create": "Create",
+    "delete": "Delete",
+    "deleteConfirm": "Delete the format “{{name}}”?"
+  },
+  "admin": {
+    "storageTitle": "Storage",
+    "readyFiles": "Ready files",
+    "totalSize": "Total size",
+    "cap": "Cache cap",
+    "noCap": "no cap",
+    "perUser": "Per-user footprint",
+    "user": "User",
+    "footprint": "Footprint",
+    "editQuota": "Edit quota",
+    "quotaTitle": "Download quota — {{email}}",
+    "maxBytes": "Storage limit (GB)",
+    "maxJobs": "Max downloads",
+    "maxConcurrent": "Max concurrent",
+    "unlimited": "Unlimited (bypass storage limit)",
+    "reset": "Reset to default",
+    "save": "Save",
+    "custom": "custom",
+    "default": "default"
+  },
+  "cols": {
+    "title": "Title",
+    "channel": "Channel",
+    "format": "Format",
+    "size": "Size",
+    "status": "Status",
+    "added": "Added",
+    "user": "User"
+  }
+}