feat: implement server password login, YouTube source, and improved bot selector UI

- **TS server password** (#7/#9): add serverPassword field across database,
  manager, bot API, and Settings UI — allows joining password-protected servers
- **YouTube audio source** (#1/#10): new YouTubeProvider using yt-dlp binary;
  adds -y flag in chat commands, /api/music supports platform=youtube,
  YouTube badge in SongCard, yt-dlp-wrap npm dependency
- **Bot selector UI** (#14): selector always visible (not just when >1 bot),
  bigger button with border and play-state indicator; per-bot URL routing at
  /bot/:id with BotRedirect view; copy-link button in dropdown

Closes #1, #7, #9, #10, #14

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -30,7 +30,8 @@
     "pino": "^10.3.1",
     "ts3-nodejs-library": "^3.5.1",
     "tweetnacl": "^1.0.3",
-    "ws": "^8.20.0"
+    "ws": "^8.20.0",
+    "yt-dlp-wrap": "^2.3.12"
   },
   "devDependencies": {
     "@types/better-sqlite3": "^7.6.13",