feat(desktop): add Text-to-Speech Phase 2 and update docs/registry
- Add configurable TTS providers: system OS voice, OpenAI TTS, ElevenLabs,
Piper (local), and OpenAI-compatible (OpenRouter/LiteLLM/WaveSpeedAI/custom).
- Add secure TTS credential storage, endpoint presets, voice/model/speed UI,
and test/stop controls in Settings.
- Wire TTS to pet/floating-chat renderer via openpets:tts-speak/audio/stop IPC.
- Respect quiet hours; validate cloud endpoints and returned audio buffers.
- Add plugin voice:speak bridge and tts-engine unit tests.
- Update FEATURES.md, FEATURES_OUR_CHANGES.md, README.md, README_OUR_CHANGES.md,
PULL_REQUEST.md, FEATURE_REGISTRY.md, and add docs/pull-requests/PR_TTS_PHASE_2.md.
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