feat(stt): add local speech-to-text with whisper.cpp

- Add stt-dictate script for push-to-talk dictation
- Add Mod+Space keybinding in i3 (hold to record, release to transcribe)
- Add whisper-cpp, alsa-utils, xdotool, libnotify packages
- Enable dunst notification daemon

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages.nix

@@ -150,6 +150,10 @@
 
     # AI
     ollama
+    whisper-cpp # STT local
+    alsa-utils # arecord pour l'enregistrement
+    xdotool # pour taper le texte transcrit
+    libnotify # notifications
 
     # Perso
     nextcloud-client