feat(ai): add MiniMax as third LLM provider

[octo-patch]

Summary

Add MiniMax as a first-class AI provider alongside OpenAI and Anthropic. MiniMax models (M2.7, M2.5, M2.5-highspeed) are fully compatible with the OpenAI chat completions API, so the existing openai-go SDK is reused with MiniMax's base URL (https://api.minimax.io/v1).

Changes

Backend (Go)

• Add GeneralAIProviderMiniMax = "minimax" constant and MiniMax-M2.7 default model
• Add NewMiniMaxClient() factory that returns an OpenAI-compatible client configured with MiniMax's API endpoint
• Route MiniMax provider through the existing OpenAI conversation flow (streaming, tool calling, thinking extraction all work out-of-the-box)
• Update NormalizeGeneralAIProvider(), IsGeneralAIProviderSupported(), DefaultGeneralAIModelByProvider(), and providerLabel() to handle MiniMax

Frontend (React/TypeScript)

• Add MiniMax option to provider dropdown in general settings UI
• Update TypeScript types (GeneralSetting, GeneralSettingUpdateRequest) to include minimax
• Add MiniMax-specific model placeholder (MiniMax-M2.7) and base URL hint (https://api.minimax.io/v1)

Tests

• 15 new unit tests in pkg/ai/config_test.go covering provider normalization, labels, client factory creation and rejection
• 12 new unit tests in pkg/model/general_setting_test.go covering provider support validation, default models, constants

Documentation

• Updated both English and Chinese READMEs to mention MiniMax alongside OpenAI and Anthropic

How to Use

1. Go to Settings > General > AI Agent
2. Select MiniMax from the Provider dropdown
3. Enter your MiniMax API key
4. The model defaults to MiniMax-M2.7 and the base URL defaults to https://api.minimax.io/v1
5. Click Save

Test Plan

• All 27 new unit tests pass (go test ./pkg/ai/... ./pkg/model/...)
• Existing tests unaffected (language detection, resource matching)
• Manual verification: configure MiniMax provider in settings UI, verify chat works with MiniMax API key
• Verify OpenAI and Anthropic providers still work after changes

Files Changed (9 files, 345 additions)

File	Description
pkg/model/general_setting.go	MiniMax provider constant, default model, normalization
pkg/model/general_setting_test.go	12 unit tests for model layer
pkg/ai/config.go	NewMiniMaxClient() factory, provider label
pkg/ai/config_test.go	15 unit tests for config layer
pkg/ai/agent.go	MiniMax routing in NewAgent()
ui/src/lib/api.ts	TypeScript type union update
ui/src/components/settings/general-management.tsx	Provider dropdown, model/URL placeholders
README.md	MiniMax mention in features
README_zh.md	MiniMax mention in features (Chinese)