feat: add strict Kokoro offline TTS

[wimi321]

Summary

This PR adds strict Kokoro zh-CN offline TTS for GoAgent teacher answers and prepares the v0.3.16 release.

Changes

• Adds a selected-provider TTS registry with no system voice, no Web Speech, no fallback chain, and no automatic provider switching.
• Adds the default kokoro-bundled provider backed by local data/tts/kokoro/zh-CN assets only.
• Adds explicit custom TTS providers for OpenAI-compatible speech, HTTP JSON, and localhost services.
• Stores custom TTS API keys through safeStorage secrets rather than plain settings.
• Adds teacher speech controls and a TTS settings panel.
• Adds Kokoro asset preparation, strict asset validation, provider-policy checks, and real offline synthesis smoke.
• Updates release packaging so GitHub Release jobs prepare Kokoro ONNX assets before building installers.
• Bumps version and multilingual release notes to v0.3.16.

Verification

• pnpm install
• pnpm check
• GOAGENT_TTS_ASSETS_STRICT=1 pnpm check:tts-assets
• GOAGENT_TTS_SMOKE_STRICT=1 pnpm smoke:tts
• pnpm check:teacher-quality
• pnpm check:release-quality

Large File Policy

The Kokoro ONNX model files are prepared by script and ignored by normal Git. This PR commits only manifest/config/tokenizer metadata and small voice files.