feat: add MiniMax as LLM and TTS provider

[octo-patch]

Summary

Add MiniMax as an alternative LLM and TTS provider, giving users access to MiniMax-M2.7 (1M context) alongside the existing OpenAI integration.

Chat Provider

• Provider selector in Chat settings to switch between OpenAI and MiniMax
• MiniMax API key input with env var (VITE_MINIMAX_API_KEY) support
• Model selection: MiniMax-M2.7 and MiniMax-M2.7-highspeed
• Temperature clamping to (0, 1] range as required by MiniMax API
• Think-tag stripping from MiniMax model responses

MiniMax TTS

• Added as 4th speech synthesis service alongside System, Azure TTS, and Amazon Polly
• Models: speech-2.8-hd (high quality) and speech-2.8-turbo (fast)
• 12 English voices with voice selector in Synthesis settings
• Shares MiniMax API key with chat provider (no duplicate configuration)

Other Changes

• i18n support in English, Chinese (zh-CN), and Spanish (es) locales
• Updated tips and README with MiniMax setup instructions
• 33 unit and integration tests (vitest)

Files Changed (18 files, 702 additions)

File	Change
src/apis/openai.ts	Provider-aware routing, temp clamping, think-tag strip
src/apis/minimaxTTS.ts	New - MiniMax TTS API client
src/store/reducer/global.ts	Add chatProvider, minimaxApiKey, minimaxModel, TTS fields
src/store/module/global.ts	Default state for MiniMax fields
src/components/Settings/ChatSection.tsx	Provider selector, MiniMax settings UI
src/components/Settings/SynthesisSection.tsx	MiniMax TTS service option
src/components/Content.tsx	Provider-based API key/host/model routing
src/utils/speechSynthesis.ts	MiniMax TTS playback integration
src/locales/{en,zh-CN,es}.json	i18n strings for MiniMax
README.md	MiniMax setup tutorial
src/tests/*.test.ts	33 tests (unit + integration)

Test Plan

• TypeScript type checking passes (npx tsc --noEmit)
• 33/33 tests pass (npx vitest run)
• Manual: select MiniMax provider, enter API key, send a chat message
• Manual: select MiniMax TTS, play a response with different voices
• Manual: verify OpenAI provider still works unchanged
• Manual: verify temperature slider caps at 1.0 when MiniMax is selected