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>

programs/i3.nix

@@ -76,6 +76,9 @@
           # Sreen brightness controls
           "XF86MonBrightnessUp" = "exec light -A 2"; # increase screen brightness
           "XF86MonBrightnessDown" = "exec light -U 2"; # decrease screen brightness
+
+          # Speech-to-text (toggle: press to start/stop)
+          "${modifier}+space" = "exec ~/.local/bin/stt-dictate toggle";
         };
 
         startup = [