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

mcp/options.go

@@ -160,3 +160,17 @@ func WithQwenConfig(apiKey string) ClientOption {
 		c.Model = DefaultQwenModel
 	}
 }
+
+// WithMiniMaxConfig sets MiniMax configuration
+//
+// Usage example:
+//
+//	client := mcp.NewClient(mcp.WithMiniMaxConfig("sk-xxx"))
+func WithMiniMaxConfig(apiKey string) ClientOption {
+	return func(c *Config) {
+		c.Provider = ProviderMiniMax
+		c.APIKey = apiKey
+		c.BaseURL = DefaultMiniMaxBaseURL
+		c.Model = DefaultMiniMaxModel
+	}
+}