FEAT: 集成 Kokoro 多语言 TTS 与文本转课时

[styayur]

Summary

• upgrade sherpa_onnx to 1.13.8 for official Kokoro v1.1 ONNX Runtime compatibility
• switch Kokoro downloads to multilingual fp32/int8 v1.1 (English + Chinese, 103 speakers)
• add text segmentation, streaming WAV assembly, SRT sentence metadata, and Text to Listening lesson creation
• add configurable 0.5x-2.0x local TTS speed
• add Local TTS as a shadowing reference source
• update docs and third-party notices

Verification

• focused TTS/Text Lesson/listen-and-repeat tests: 277 passed, 1 real-model test skipped without env
• real Kokoro int8 Chinese/English mixed synthesis passed on Windows test runner
• targeted flutter analyze: no issues
• full flutter analyze: 39 existing non-fatal info/warning items, no errors
• full flutter test still reports unrelated pre-existing environment/native-plugin failures; focused regression suite passes
• Windows/Android builds could not run locally: symlink/Developer Mode and Android SDK are unavailable

[styayur]

Implementation and source archive are published in the fork release: https://github.com/styayur/Echo-Loop/releases/tag/v1.0.39. Upstream branch/tag publishing is blocked because the authenticated account has read-only permission on echo-loop/Echo-Loop, so this PR is the upstream integration path.