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

api/backtest.go

@@ -832,6 +832,8 @@ func (s *Server) hydrateBacktestAIConfig(cfg *backtest.BacktestConfig) error {
 			provider = "google"
 		} else if strings.Contains(modelNameLower, "deepseek") {
 			provider = "deepseek"
+		} else if strings.Contains(modelNameLower, "minimax") {
+			provider = "minimax"
 		} else if model.CustomAPIURL != "" {
 			provider = "custom"
 		} else {