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/audio/player.ts

@@ -302,6 +302,13 @@ export class AudioPlayer extends EventEmitter {
 
   seek(seconds: number): void {
     if (!this.currentUrl) return;
+    // Reject NaN/Infinity/negative — the HTTP layer validates too, but a
+    // bad value here would poison seekOffset and leave getElapsed()
+    // returning NaN until the track ends.
+    if (!Number.isFinite(seconds) || seconds < 0) {
+      this.logger.warn({ seek: seconds }, "Ignoring invalid seek position");
+      return;
+    }
     this.logger.info({ seek: seconds }, "Seeking");
     this.play(this.currentUrl, seconds);
   }

src/audio/queue.test.ts

@@ -89,6 +89,49 @@ describe("PlayQueue", () => {
     expect(queue.list()[1].id).toBe("3");
   });
 
+  it("removing a song before current shifts current index", () => {
+    queue.setMode(PlayMode.Sequential);
+    queue.add(makeSong("A"));
+    queue.add(makeSong("B"));
+    queue.add(makeSong("C"));
+    queue.playAt(2); // playing C at index 2
+    queue.remove(0); // remove A (before current)
+    expect(queue.current()?.id).toBe("C"); // still on C
+    expect(queue.getCurrentIndex()).toBe(1);
+  });
+
+  it("removing the currently-playing song lets next() advance to the shifted song", () => {
+    queue.setMode(PlayMode.Sequential);
+    queue.add(makeSong("A"));
+    queue.add(makeSong("B"));
+    queue.add(makeSong("C"));
+    queue.add(makeSong("D"));
+    queue.playAt(2); // playing C
+    queue.remove(2); // remove C — D shifts into slot 2
+    // Before the fix this returned null (D was silently skipped)
+    expect(queue.next()?.id).toBe("D");
+  });
+
+  it("removing the only song clears the queue", () => {
+    queue.add(makeSong("only"));
+    queue.playAt(0);
+    queue.remove(0);
+    expect(queue.size()).toBe(0);
+    expect(queue.current()).toBeNull();
+    expect(queue.next()).toBeNull();
+  });
+
+  it("removing the last song while playing it advances to null in sequential mode", () => {
+    queue.setMode(PlayMode.Sequential);
+    queue.add(makeSong("A"));
+    queue.add(makeSong("B"));
+    queue.playAt(1); // playing B (last)
+    queue.remove(1);
+    expect(queue.size()).toBe(1);
+    // currentIndex moved to 0, so next() should try to advance past the end
+    expect(queue.next()).toBeNull();
+  });
+
   it("clears all songs", () => {
     queue.add(makeSong("1"));
     queue.add(makeSong("2"));

src/audio/queue.ts

@@ -36,9 +36,14 @@ export class PlayQueue {
     if (index < this.currentIndex) {
       this.currentIndex--;
     } else if (index === this.currentIndex) {
-      if (this.currentIndex >= this.songs.length) {
-        this.currentIndex = this.songs.length - 1;
-      }
+      // Move the pointer back by one so next() in sequential mode advances
+      // to the song that shifted into the removed slot. Without this, the
+      // shifted song is silently skipped because current() incorrectly
+      // returns it (even though the player is still on the removed song)
+      // and next() then increments past it. currentIndex may become -1,
+      // which is fine — it represents "no current song" and next() will
+      // pick index 0.
+      this.currentIndex--;
     }
 
     return removed;