Rework the welcome screen around the shared new-chat composer, renewed quick actions, discovery cards, OAuth accounts, and dedicated system resources panel.
Restore static button colors and neutral surfaces, move microphone state feedback onto the icon, and keep Rubik/code-font composer behavior.
Add focused static regressions for the welcome composer, OAuth discovery panel, and speech button contract.
Split the legacy core speech stack into two built-in, independently toggleable plugins: `_kokoro_tts` for TTS and `_whisper_stt` for STT.
This refactor keeps dependency installation and bootstrap concerns in Docker/bootstrap/preload, while moving speech-specific tooling, APIs, prompts, UI, and runtime behavior into the plugins. Core now exposes engine-agnostic `tts-service` and `stt-service` brokers, with browser-native TTS preserved as the fallback when Kokoro is disabled.
Included in this change:
- add built-in `_kokoro_tts` plugin with plugin-owned synth API, config, status UI, and provider registration
- add built-in `_whisper_stt` plugin with plugin-owned transcribe API, mic runtime, device UI, prompt injection, and provider registration
- remove legacy core speech APIs/helpers/settings/UI and delete unused `webui/js/speech_browser.js`
- replace the old hardcoded speech settings section with a generic voice surface backed by plugin extensions
- update preload/docs/tests to match the new plugin-owned speech architecture
Behavioral intent:
- both plugins are built-in but not `always_enabled`
- users can now hot-switch TTS and STT independently
- browser TTS remains available when `_kokoro_tts` is off
- Whisper mic UI only appears when `_whisper_stt` is enabled
