Adds a focused Windows node text-to-speech capability as the first stable voice-support primitive.
- adds the shared `tts.speak` capability and MCP/gateway documentation
- wires Windows and ElevenLabs TTS behind opt-in tray settings
- protects the ElevenLabs API key with DPAPI
- adds shared and tray tests for capability behavior, settings, and ElevenLabs requests
This lands the focused TTS foundation from the broader Voice Mode discussion in #120 so remaining voice UX/STT/repeater work can build on top in smaller follow-up PRs.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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