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/registry.go

@@ -0,0 +1,20 @@
+package mcp
+
+// providerRegistry maps provider names to factory functions.
+var providerRegistry = map[string]func(...ClientOption) AIClient{}
+
+// RegisterProvider registers a provider factory function.
+// Called by provider/payment sub-packages in their init() functions.
+func RegisterProvider(name string, factory func(...ClientOption) AIClient) {
+	providerRegistry[name] = factory
+}
+
+// NewAIClientByProvider creates an AIClient by provider name using the registry.
+// Returns nil if the provider is not registered.
+func NewAIClientByProvider(name string, opts ...ClientOption) AIClient {
+	factory, ok := providerRegistry[name]
+	if !ok {
+		return nil
+	}
+	return factory(opts...)
+}