Add MiniMax AI as a built-in LLM supplier alongside existing providers
(OpenAI, DeepSeek, Gemini, Kimi, etc.) via OpenAI-compatible API.
Changes:
- Add MiniMax supplier entry (id=13) in supplier.ts with M2.7, M2.5,
M2.5-highspeed models and temperature range [0, 1]
- Add MiniMax icon (96x96 PNG) to frontend assets
- Add i18n translations for MiniMax in en, zh-CN, ko-KR locales
- Add supported LLM providers table to both README files
- Add 24 unit tests + 3 integration tests
