feat: playlist save/load, queue durations, whisper bias, greet fix

- Move TTS greeting from mumble._play_greet to voice.on_connected
  (fires once on first connect, gated on _is_audio_ready)
- Add initial_prompt multipart field to Whisper STT for trigger word
  bias (auto-generated from trigger config, overridable)
- Enhanced !queue: elapsed/total on now-playing, per-track durations,
  footer with track count and total time
- New !playlist command: save/load/list/del named playlists via
  bot.state persistence (playlist:<name> keys)
- Fix duck floor test (1% -> 2% to match default change)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USAGE.md

@@ -1623,13 +1623,17 @@ and voice transmission.
 !prev                Go back to the previous track (fade-out)
 !seek <offset>       Seek to position (1:30, 90, +30, -30)
 !resume              Resume last stopped/skipped track from saved position
-!queue               Show queue
+!queue               Show queue (with durations + totals)
 !queue <url>         Add to queue (alias for !play)
 !np                  Now playing
 !volume [0-100]      Get/set volume (persisted across restarts)
 !keep                Keep current track's audio file (with metadata)
 !kept [rm <id>|clear|repair] List, remove, clear, or repair kept files
 !testtone            Play 3-second 440Hz test tone
+!playlist save <name>   Save current + queued tracks as named playlist
+!playlist load <name>   Append saved playlist to queue, start if idle
+!playlist list          Show saved playlists with track counts
+!playlist del <name>    Delete a saved playlist
 ```
 
 - Queue holds up to 50 tracks
@@ -1712,7 +1716,7 @@ volume gradually restores to the user-set level in small steps.
 !duck                  Show ducking status and settings
 !duck on               Enable voice ducking
 !duck off              Disable voice ducking
-!duck floor <0-100>    Set floor volume % (default: 1)
+!duck floor <0-100>    Set floor volume % (default: 2)
 !duck silence <sec>    Set silence timeout in seconds (default: 15)
 !duck restore <sec>    Set restore ramp duration in seconds (default: 30)
 ```