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>

scripts/test_bot_remove.py

@@ -0,0 +1,99 @@
+"""Regression: DELETE /api/bot/:id must broadcast botRemoved so the UI drops the row.
+
+Creates an ephemeral bot, opens the dropdown, deletes the bot via API, and
+asserts the row disappears without any page reload. Does not touch any
+existing user bot.
+"""
+import time
+import requests
+from playwright.sync_api import sync_playwright
+
+BASE = "http://localhost:3000"
+EPHEMERAL_NAME = "rmbot_test"
+EPHEMERAL_NICK = "RmBotTest"
+
+
+def api(path, method="GET", **kw):
+    r = getattr(requests, method.lower())(f"{BASE}{path}", timeout=10, **kw)
+    r.raise_for_status()
+    return r.json() if r.text else None
+
+
+def cleanup():
+    for b in api("/api/bot/")["bots"]:
+        if b["name"] == EPHEMERAL_NAME:
+            try:
+                api(f"/api/bot/{b['id']}", method="DELETE")
+            except Exception:
+                pass
+
+
+def main():
+    cleanup()
+    new = api(
+        "/api/bot/",
+        method="POST",
+        json={
+            "name": EPHEMERAL_NAME,
+            "serverAddress": "127.0.0.1",
+            "serverPort": 9987,
+            "nickname": EPHEMERAL_NICK,
+            "autoStart": False,
+        },
+    )
+    bot_id = new["id"]
+    print(f"[setup] created ephemeral bot {bot_id[:8]}")
+
+    try:
+        with sync_playwright() as p:
+            browser = p.chromium.launch(headless=True)
+            try:
+                page = browser.new_page(viewport={"width": 1440, "height": 900})
+                page.goto(BASE)
+                page.wait_for_load_state("networkidle")
+                time.sleep(0.8)
+
+                # Open dropdown and confirm the new bot row is present
+                page.locator(".bot-selector-btn").click()
+                page.wait_for_selector(".bot-dropdown")
+                rows_before = page.locator(".bot-dropdown-row").count()
+                print(f"[ui] dropdown rows before remove: {rows_before}")
+
+                # Match the ephemeral row by its name text
+                present = (
+                    page.locator(".bot-dropdown-row", has_text=EPHEMERAL_NAME).count()
+                )
+                assert present == 1, f"ephemeral row not found (got {present})"
+
+                # Delete via API
+                api(f"/api/bot/{bot_id}", method="DELETE")
+                print("[api] deleted bot")
+
+                # Wait up to 4s for UI to drop the row
+                removed = False
+                for _ in range(40):
+                    if (
+                        page.locator(
+                            ".bot-dropdown-row", has_text=EPHEMERAL_NAME
+                        ).count()
+                        == 0
+                    ):
+                        removed = True
+                        break
+                    time.sleep(0.1)
+
+                rows_after = page.locator(".bot-dropdown-row").count()
+                print(f"[ui] dropdown rows after remove: {rows_after}")
+                assert removed, "ephemeral row did not disappear from UI after DELETE"
+                assert rows_after == rows_before - 1, (
+                    f"row count mismatch: before={rows_before} after={rows_after}"
+                )
+                print("[PASS] bot removal propagates to UI via WS")
+            finally:
+                browser.close()
+    finally:
+        cleanup()
+
+
+if __name__ == "__main__":
+    main()