Wire watch history, resume positions and persistent queue

WatchPage now records every played video in watch_history, saves the
playback position every 5 seconds and on stop/end (completed at >=95%),
and seeks back on replay when player.resume is "auto" - resume kicks in
between 30 seconds and 95% of the duration. The play queue persists
across restarts and reorders/removals write through immediately; the
main window's closeEvent flushes position and queue and releases libmpv.

PlayerPanel gains an automatic stall-retry: YouTube's CDN intermittently
serves stalled streams to non-browser clients (reproduced ~1/3 of
attempts headless with identical code), and a reload re-resolves onto a
healthy node - two retries after 25s of no playback, then a user-facing
error.

ytsage_constants now prepends the managed-binaries dir to PATH so
yt-dlp subprocesses (including mpv's ytdl_hook) can find the managed
Deno runtime - previously nothing exported APP_BIN_DIR, so the deno
integration silently never worked.

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

ytsage/languages/en.json

@@ -647,7 +647,8 @@
     "now_playing": "Now Playing",
     "queue": "Queue",
     "play": "Play",
-    "pause": "Pause"
+    "pause": "Pause",
+    "stream_stalled": "Stream failed to start after several attempts - YouTube may be throttling. Try again or pick a lower quality."
   },
   "cards": {
     "play": "▶ Play",