feat: voice language selection in chat, per-voice TTS and STT locale

[grunjol]

Lets users select a specific TTS voice and STT locale from Settings
instead of hardcoding Australian English for both.

Changes

settings_screen.dart — adds _VoicePicker widget in the Voice
section. It queries flutter_tts.getVoices() to list available voices,
sorted by locale. Falls back to getLanguages() if no voices are
installed. Selection is persisted as voice_name and voice_locale in
SharedPreferences. The picker only shows voices whose locale is
supported by the device's speech-to-text engine.

chat_screen.dart — reads the saved voice config in _initVoice().
If a specific voice is chosen, calls setVoice() for TTS and derives
the STT locale from the voice's locale. If the voice name equals the
locale (language-only fallback), calls setLanguage() instead. Passes
localeId to SpeechListenOptions so dictation uses the chosen
language. When no voice is configured, leaves localeId null to use
the device default.

[rusty4444]

Hermes Agent Automated Review — PR #70

Verdict: APPROVE ✅

Summary

Clean implementation of voice language selection. Allows users to pick TTS voices from device settings, persists the choice with SharedPreferences, and passes the locale to speech-to-text.

✅ Looks Good

• Proper null safety and fallback to getLanguages when getVoices returns empty
• Gender-aware voice labels for better UX
• Clean separation into _VoicePicker widget in settings
• STT locale properly derived from voice locale with underscore conversion
• Auto (device default) fallback when no voice selected

💡 Suggestions (non-blocking)

• Consider caching SharedPreferences instance instead of calling getInstance() multiple times

---

Reviewed by Hermes Agent — July 8, 2026