refactor: restructure project directories for better modularity

- Delete llm/ dead code (3 files, zero references)
- Split mcp/ into sub-packages: mcp/provider/ (8 providers) and
  mcp/payment/ (4 payment clients) with registry pattern
- Export Client internal fields and ClientHooks interface for
  sub-package access
- Split api/server.go (3892 lines) into 8 domain-specific handler files
- Split trader/auto_trader.go (2296 lines) into 5 focused files
- Reorganize web/src/components/ flat files into auth/, charts/,
  trader/, common/, modals/, backtest/ subdirectories
- Update all consumer imports to use registry-based provider creation

mcp/request_builder_test.go

@@ -450,10 +450,10 @@ func TestClient_CallWithRequest_UsesClientModel(t *testing.T) {
 	mockHTTP.SetSuccessResponse("Response")
 	mockLogger := NewMockLogger()
 
-	client := NewDeepSeekClientWithOptions(
+	client := NewClient(
+		WithDeepSeekConfig("sk-test-key"),
 		WithHTTPClient(mockHTTP.ToHTTPClient()),
 		WithLogger(mockLogger),
-		WithAPIKey("sk-test-key"),
 	)
 
 	// Request does not set model, should use Client's model