feat: live macOS dictation — push-to-talk mic into the realtime ASR session

- /dictate + ctrl+shift+d toggle one shared session: press to start, press again
  to stop, flush and paste into the prompt (a terminal gives no key-release event
  for hold-to-talk without the Kitty keyboard protocol)
- client.recordMic(): spawns ffmpeg AVFoundation capture and appends s16le frames
  as they arrive (no pacer — the mic is already 1x paced); stop() = commit + the
  same quiescence drain transcribe() uses, since is_last_result never flips true
- enumerates mics by slicing the "AVFoundation audio devices:" block off stderr
  (ffmpeg prints the table there, then fails opening the dummy input)
- --device takes an index or name, --lang / ASR_DICTATE_LANGUAGE, --submit sends
  instead of pasting; asr_health now reports the mic inputs it enumerated
- capture death surfaces via the emptyGraceMs path with the TCC hint (System
  Settings → Privacy & Security → Microphone) when ffmpeg exits non-zero at 0 bytes
- env.sh sources the gitignored .env so a shell gets ASR_NIM_URL / ASR_API_KEY

.gitignore

@@ -6,5 +6,6 @@ yarn-error.log
 *.mp3
 *.mp4
 asr.json
+.env
 .DS_Store
 *.log