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>

src/web/api/music.ts

@@ -1,5 +1,6 @@
 import { Router } from "express";
 import type { MusicProvider } from "../../music/provider.js";
+import { YouTubeProvider } from "../../music/youtube.js";
 import type { Logger } from "../../logger.js";
 
 export function createMusicRouter(
@@ -9,9 +10,11 @@ export function createMusicRouter(
   logger: Logger
 ): Router {
   const router = Router();
+  const youtubeProvider: MusicProvider = new YouTubeProvider();
 
   function getProvider(platform?: string): MusicProvider {
     if (platform === "bilibili") return bilibiliProvider;
+    if (platform === "youtube") return youtubeProvider;
     return platform === "qq" ? qqProvider : neteaseProvider;
   }