feat: add MiniMax provider support (#1406)

Add MiniMax as a new AI model provider with OpenAI-compatible API.

Supported models:
- MiniMax-M2.5 (default) - Peak Performance, Ultimate Value
- MiniMax-M2.5-highspeed - Same performance, faster and more agile

Changes:
- Add MiniMax client (mcp/minimax_client.go) with OpenAI-compatible API
- Add comprehensive unit tests (mcp/minimax_client_test.go)
- Add WithMiniMaxConfig option (mcp/options.go)
- Register MiniMax provider in trader, debate engine, backtest, and API
- Add MiniMax to frontend provider config and model icons
- Add MiniMax SVG icon

API Base URL: https://api.minimax.io/v1

web/src/pages/DebateArenaPage.tsx

@@ -104,6 +104,7 @@ function AIAvatar({ name, size = 24 }: { name: string; size?: number }) {
     kimi: { bg: 'bg-purple-500', text: 'text-white', letter: 'K' },
     qwen: { bg: 'bg-indigo-500', text: 'text-white', letter: 'Q' },
     openai: { bg: 'bg-emerald-600', text: 'text-white', letter: 'O' },
+    minimax: { bg: 'bg-red-500', text: 'text-white', letter: 'M' },
     gpt: { bg: 'bg-emerald-600', text: 'text-white', letter: 'O' },
   }
   const lower = name.toLowerCase()