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

debate/engine.go

@@ -97,6 +97,8 @@ func (e *DebateEngine) InitializeClients(participants []*store.DebateParticipant
 			client = mcp.NewGrokClient()
 		case "kimi":
 			client = mcp.NewKimiClient()
+		case "minimax":
+			client = mcp.NewMiniMaxClient()
 		default:
 			client = mcp.New()
 		}