fix(tts): restore Supertonic 2

[AnthonyRonning]

Summary

• restore the Supertonic 2 model set and synthesis pipeline
• remove Supertonic 3-only language, compatibility, download, and chunked-playback code
• retain speech-speed controls, full-response pre-synthesis, preparation/playback cancellation UI, and iOS background audio support
• skip artifact migration because Supertonic 3 has not shipped

Validation

• frontend tests: 244 passed
• frontend build, typecheck, formatting, and lint completed; lint reports 12 pre-existing warnings and no errors
• native tests: 204 passed, 1 ignored, including 6 focused TTS tests
• built and launched the workspace-specific macOS app against the full local OpenSecret, billing, Postgres, and Continuum stack
• signed in with a local Pro fixture and completed real chat requests
• downloaded the expected Supertonic 2 model set through the UI (about 252 MB advertised; 271 MiB on disk)
• verified speed adjustment from 1.2x to 1.5x and reset
• verified native synthesis completion for short and longer responses, playback state transitions, stop, and preparation-cancel UI

Notes

• GUI and native logs were verified, but audio quality was not audibly evaluated
• the local debug app and DMG were produced successfully; the build command only failed afterward because no updater signing private key is configured locally
• iPhone validation remains the release gate

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