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

trader/auto_trader.go

@@ -201,6 +201,11 @@ func NewAutoTrader(config AutoTraderConfig, st *store.Store, userID string) (*Au
 		mcpClient.SetAPIKey(config.CustomAPIKey, config.CustomAPIURL, config.CustomModelName)
 		logger.Infof("🤖 [%s] Using OpenAI", config.Name)
 
+	case "minimax":
+		mcpClient = mcp.NewMiniMaxClient()
+		mcpClient.SetAPIKey(config.CustomAPIKey, config.CustomAPIURL, config.CustomModelName)
+		logger.Infof("🤖 [%s] Using MiniMax AI", config.Name)
+
 	case "qwen":
 		mcpClient = mcp.NewQwenClient()
 		apiKey := config.QwenKey