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>

web/src/components/SongCard.vue

@@ -7,8 +7,8 @@
         <span class="song-name">{{ song.name }}</span>
         <span
           class="platform-badge"
-          :class="song.platform === 'bilibili' ? 'badge-bilibili' : song.platform === 'qq' ? 'badge-qq' : 'badge-netease'"
-        >{{ song.platform === 'bilibili' ? 'B站' : song.platform === 'qq' ? 'QQ' : '网易云' }}</span>
+          :class="song.platform === 'bilibili' ? 'badge-bilibili' : song.platform === 'qq' ? 'badge-qq' : song.platform === 'youtube' ? 'badge-youtube' : 'badge-netease'"
+        >{{ song.platform === 'bilibili' ? 'B站' : song.platform === 'qq' ? 'QQ' : song.platform === 'youtube' ? 'YouTube' : '网易云' }}</span>
       </div>
       <div class="song-artist">{{ song.artist }}</div>
     </div>
@@ -118,6 +118,11 @@ function formatDuration(seconds: number): string {
   color: #00a1d6;
 }
 
+.badge-youtube {
+  background: rgba(255, 0, 0, 0.12);
+  color: #ff0000;
+}
+
 .song-artist {
   font-size: 12px;
   color: var(--text-secondary);