fix: harden bot lifecycle, validate HTTP inputs, make YouTube truly optional

Major bug fixes and corner-case hardening across the backend, plus a
comprehensive feature test suite. All 94 unit tests + 51 integration
tests pass against a local TS3 server.

Lifecycle & state consistency
-----------------------------
- Bug A: startBot() now wraps connect() in a 15s deadline. A hung TS
  handshake no longer blocks the /start HTTP call forever; the failing
  instance is torn down and the caller gets a clean 500.
- Bug B: executeCommand rejects audio-dispatching commands (play, add,
  next, skip, prev, playlist, album, fm) when the bot is disconnected.
  Config-only commands (vol, mode, clear, stop, queue, now, lyrics)
  still work so the UI stays usable while offline.
- Bug C: the tsClient 'disconnected' handler always clears player state
  now, even when connect() never completed. A separate disconnectEmitted
  flag guards duplicate external event emission. Previously an orphaned
  connect attempt that idle-timed-out would leave playing=true forever.
- resolveAndPlay re-checks this.connected AFTER the URL-resolve await so
  a stop() during the network call can't spawn ffmpeg on a disconnected
  bot.
- connect() throws if disconnect() fired during the handshake await,
  preventing a concurrent stop from being overwritten by a late connected
  flag flip.
- startBot always disconnects the outgoing BotInstance before creating
  a replacement, covering the mid-handshake case where isConnected()
  still returned false but the library client was live.
- startBot now reuses the stored identity so server groups granted to
  the bot survive restarts (was regenerating a fresh UID each time).

WebSocket reliability
---------------------
- BotManager extends EventEmitter and emits 'botInstance' whenever a
  new instance is created. websocket.ts listens and re-attaches its
  stateChange / connected / disconnected listeners immediately, fixing
  the bug where player-bar UI never updated until manual refresh.
- attachedBots map now stores the BotInstance reference and detaches
  stale listeners when the instance is replaced. Safety-net interval
  (5s) also reconciles to catch anything missed.
- removeBot emits 'botInstanceRemoved' -> WS broadcasts a new
  {type:"botRemoved", botId} message. Client drops the bot from its
  local store instead of showing it as permanently offline.

HTTP input validation
---------------------
- /volume rejects non-number, NaN, Infinity, and out-of-range values
  with a proper 400 instead of a 200 OK wrapping a usage-text string.
- /mode rejects anything not in {seq, loop, random, rloop} with 400.
- /seek rejects NaN / Infinity / negative (previously NaN slipped
  through typeof==="number" and poisoned seekOffset).
- /play-at validates index < queue.size() BEFORE stopping current
  playback (was silently killing the current song on invalid input).
- /play, /add, /playlist, /play-by-id, /add-by-id, /play-playlist
  all honour platform=youtube now (previously fell through to netease
  and silently played the wrong platform).

YouTube made truly optional
---------------------------
- Lazy checkYtDlpAvailable() runs `yt-dlp --version` once, caches only
  positive results so users can install yt-dlp mid-run and have it
  picked up without a restart.
- getAuthStatus() returns loggedIn=false with nickname
  "YouTube (yt-dlp not installed)" when the binary is missing. UI can
  grey out YouTube instead of silently returning empty searches.
- findYtDlp() picks .exe on win32 and bare binary elsewhere.
- /auth/status?platform=youtube now routes to the YouTube provider
  instead of falling through to NetEase and leaking the NetEase
  user's nickname + avatar.
- /auth/cookie rejects platform=youtube with 400 instead of clobbering
  the NetEase cookie entry.
- README documents yt-dlp install paths (bin/ local vs PATH) and adds
  a dedicated "Optional: YouTube source" section.

Bot Selector UI
---------------
- New power button in each row of the dropdown with play-state-aware
  styling: disabled + wait-cursor during API call, green highlight when
  connected, greys out when the bot is offline.
- Dropdown always visible when >=1 bot exists, bigger font + padding.

Queue correctness
-----------------
- PlayQueue.remove(current) now decrements currentIndex so next() in
  sequential mode advances to the shifted song. Previously removing
  the currently-playing track silently skipped the next track because
  current() falsely reported it as active and next() then incremented
  past it.

Vote-skip hardening
-------------------
- cmdVote: needed threshold is Math.max(1, ceil(users/2)) so a single
  voter in an empty channel can't unanimously pass a vote with
  needed=0.
- resolveAndPlay clears voteSkipUsers on every new track load so votes
  can't leak across songs via cmdPlay/cmdPlaylist/cmdAlbum/cmdFm paths.

cmdAdd parity
-------------
- cmdAdd auto-plays the newly-added song if the player was idle,
  matching /api/player/:id/add-by-id behaviour. Previously add'ing to
  an empty queue on a connected+idle bot silently enqueued without
  starting playback.

Test suite
----------
- scripts/test_full_feature.py — 51 tests across 10 groups exercising
  every HTTP endpoint, WebSocket broadcasts, all music providers, bot
  lifecycle, disconnected-state corners, seek validation, input
  validation, and the main race conditions. Captures and restores the
  target bot's initial state. Resilient to TS3 anti-flood via retry
  with exponential backoff. Runs against a real local TS3 server.
- scripts/test_rapid_cycle.py — Bugs A/B/C regressions
- scripts/test_corner_cases.py — disconnect-during-connect race, config
  commands while disconnected, etc.
- scripts/test_more_corners.py — resolveAndPlay race, seek NaN
- scripts/test_power_button.py — E2E for the new power button
- scripts/test_bot_remove.py — E2E for WS botRemoved broadcast
- scripts/test_playbar.py — player bar auto-show regression (updated
  to restore bot state on exit)
- scripts/test_multibot.py — two-bot concurrent playback monitor
- src/audio/queue.test.ts — 4 new vitest cases for remove() edge cases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/music/youtube.ts

@@ -21,17 +21,54 @@ const __dirname = dirname(fileURLToPath(import.meta.url));
 
 /** Resolve the yt-dlp binary path. Checks the project bin/ dir first, then PATH. */
 function findYtDlp(): string {
+  const exe = process.platform === "win32" ? "yt-dlp.exe" : "yt-dlp";
   const candidates = [
-    join(__dirname, "..", "..", "bin", "yt-dlp.exe"),
+    join(__dirname, "..", "..", "bin", exe),
     join(__dirname, "..", "..", "bin", "yt-dlp"),
-    "yt-dlp",
-    "yt-dlp.exe",
+    exe,
   ];
   for (const c of candidates) {
-    if (!c.includes(join("bin", "yt-dlp")) || existsSync(c)) return c;
-    // For PATH entries (no directory prefix), fall through to let execFile try
+    // Absolute/relative paths: only return if the file exists.
+    // Bare names: return and let execFile resolve via PATH.
+    const isBinPath = c.includes(join("bin", "yt-dlp"));
+    if (!isBinPath || existsSync(c)) return c;
   }
-  return "yt-dlp";
+  return exe;
+}
+
+/**
+ * Availability check for yt-dlp. Runs `yt-dlp --version` and caches only
+ * the positive result — if the binary is missing, subsequent calls retry
+ * so the user can install yt-dlp while the server is running and pick it
+ * up without a restart. Used by getAuthStatus() so the UI can reflect
+ * whether YouTube is actually usable.
+ */
+let cachedAvailable = false;
+let pendingCheck: Promise<boolean> | null = null;
+async function checkYtDlpAvailable(): Promise<boolean> {
+  if (cachedAvailable) return true;
+  if (pendingCheck) return pendingCheck;
+  pendingCheck = (async () => {
+    try {
+      await execFileAsync(findYtDlp(), ["--version"], {
+        timeout: 5_000,
+        maxBuffer: 1024,
+      });
+      cachedAvailable = true;
+      return true;
+    } catch {
+      return false;
+    } finally {
+      pendingCheck = null;
+    }
+  })();
+  return pendingCheck;
+}
+
+/** Force re-detection on the next call (for tests). */
+export function resetYtDlpAvailabilityCache(): void {
+  cachedAvailable = false;
+  pendingCheck = null;
 }
 
 async function runYtDlp(args: string[], timeoutMs = 30_000): Promise<string> {
@@ -179,6 +216,17 @@ export class YouTubeProvider implements MusicProvider {
   getCookie(): string { return ""; }
 
   async getAuthStatus(): Promise<AuthStatus> {
-    return { loggedIn: true, nickname: "YouTube (yt-dlp)" };
+    // YouTube has no login concept via yt-dlp, so "loggedIn" here means
+    // "yt-dlp binary is reachable and responds to --version". The UI can
+    // use this flag to grey out YouTube when the optional dependency is
+    // missing, instead of silently returning empty search results.
+    const available = await checkYtDlpAvailable();
+    if (available) {
+      return { loggedIn: true, nickname: "YouTube (yt-dlp)" };
+    }
+    return {
+      loggedIn: false,
+      nickname: "YouTube (yt-dlp not installed)",
+    };
   }
 }