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/providers.go

@@ -0,0 +1,31 @@
+package mcp
+
+// Provider name constants — kept in the mcp package so that client.go can
+// reference them for default configuration without importing sub-packages.
+// Provider sub-packages re-use these same values.
+const (
+	ProviderDeepSeek = "deepseek"
+	ProviderOpenAI   = "openai"
+	ProviderClaude   = "claude"
+	ProviderQwen     = "qwen"
+	ProviderGemini   = "gemini"
+	ProviderGrok     = "grok"
+	ProviderKimi     = "kimi"
+	ProviderMiniMax  = "minimax"
+
+	ProviderBlockRunBase = "blockrun-base"
+	ProviderBlockRunSol  = "blockrun-sol"
+	ProviderClaw402      = "claw402"
+
+	// Default DeepSeek configuration (used as fallback in NewClient)
+	DefaultDeepSeekBaseURL = "https://api.deepseek.com"
+	DefaultDeepSeekModel   = "deepseek-chat"
+
+	// Default Qwen configuration (used by WithQwenConfig convenience option)
+	DefaultQwenBaseURL = "https://dashscope.aliyuncs.com/compatible-mode/v1"
+	DefaultQwenModel   = "qwen3-max"
+
+	// Default MiniMax configuration (used by WithMiniMaxConfig convenience option)
+	DefaultMiniMaxBaseURL = "https://api.minimax.io/v1"
+	DefaultMiniMaxModel   = "MiniMax-M2.5"
+)