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/hooks.go

@@ -0,0 +1,37 @@
+package mcp
+
+import "net/http"
+
+// ClientHooks is the dispatch interface used to implement per-provider
+// polymorphism without Go's lack of virtual methods.
+//
+// Each method can be overridden by an embedding struct (e.g. provider.ClaudeClient).
+// The base *Client provides OpenAI-compatible defaults; providers with a
+// different wire format (Anthropic, Gemini native, etc.) override only what
+// differs.  All call-path methods in client.go invoke these via c.Hooks so
+// that the override is always picked up at runtime.
+type ClientHooks interface {
+	// ── Simple CallWithMessages path ────────────────────────────────────────
+	Call(systemPrompt, userPrompt string) (string, error)
+	BuildMCPRequestBody(systemPrompt, userPrompt string) map[string]any
+
+	// ── Shared request plumbing ─────────────────────────────────────────────
+	BuildUrl() string
+	BuildRequest(url string, jsonData []byte) (*http.Request, error)
+	SetAuthHeader(reqHeaders http.Header)
+	MarshalRequestBody(requestBody map[string]any) ([]byte, error)
+
+	// ── Advanced (Request-object) path ──────────────────────────────────────
+	// BuildRequestBodyFromRequest converts a *Request into the provider's
+	// native wire-format map.
+	BuildRequestBodyFromRequest(req *Request) map[string]any
+
+	// ParseMCPResponse extracts the plain-text reply from a non-streaming
+	// response body.
+	ParseMCPResponse(body []byte) (string, error)
+
+	// ParseMCPResponseFull extracts both text and tool calls.
+	ParseMCPResponseFull(body []byte) (*LLMResponse, error)
+
+	IsRetryableError(err error) bool
+}