Implement channel and playlist browsing

BrowsePage accepts pasted or routed channel/playlist URLs:
- Channels get Videos / Shorts / Live sub-tabs mapped to the channel's
  /videos, /shorts and /streams listings, lazily fetched 24 at a time
  with -I range pagination; channel title and id resolve from a cheap
  -I 1:1 metadata fetch. Subscribe emits subscribeRequested for the
  Feed page to wire up.
- Playlists get a single grid with Play all (bulk-enqueues into the
  Watch queue) and a Download button that deep-links the playlist into
  the Downloads tab.

Workers are parented to their pages and fetch errors surface as status
text instead of crashing (verified against a channel with no videos
tab). fetch_flat_info gains an items="1:1" limiter so metadata probes
no longer enumerate whole channels.

Verified live: kurzgesagt channel (24 cards, title+id resolved) and a
17-video playlist with Play-all queueing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ytsage/languages/en.json

@@ -674,7 +674,19 @@
     "clear_queue": "Clear queue"
   },
   "browse": {
-    "coming_soon": "Channel & playlist browsing coming soon"
+    "placeholder": "Paste a channel or playlist URL...",
+    "open": "Open",
+    "subscribe": "Subscribe",
+    "unsubscribe": "Unsubscribe",
+    "play_all": "Play all",
+    "download_playlist": "Download",
+    "tab_videos": "Videos",
+    "tab_shorts": "Shorts",
+    "tab_live": "Live",
+    "loading": "Loading...",
+    "entry_count": "{count} videos",
+    "failed": "Failed to load: {error}",
+    "hint": "Open a channel (youtube.com/@name) or playlist URL, or navigate here from Search."
   },
   "feed": {
     "coming_soon": "Subscriptions feed coming soon"