refactor: split monolith into typed modules with retry logic

- models.py: ProxyInfo and DNSServerConfig dataclasses with type hints
- fetcher.py: subscription fetcher with exponential backoff retries (3 attempts) plus VLESS URL parser
- config_builder.py: sing-box config builder decomposed into focused helpers
- main.py: orchestrator with setup_logging(), print_banner(), pick_node()

models.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+"""Data models for Sub2SOCKS."""
+
+from dataclasses import dataclass
+from typing import Any, Dict, List, Optional
+
+
+@dataclass
+class ProxyInfo:
+    """Parsed VLESS proxy configuration."""
+    uuid: str
+    server: str
+    port: int
+    security: str = "tls"
+    type: str = "tcp"
+    sni: str = ""
+    alpn: str = ""
+    pbk: str = ""
+    sid: str = ""
+    spx: str = ""
+    path: str = "/"
+    serviceName: str = ""
+    host: str = ""
+    flow: str = ""
+
+
+@dataclass
+class DNSServerConfig:
+    """Sing-box DNS server configuration."""
+    server_type: str  # udp, tls, https
+    server: str
+    server_port: int = 53
+    tls_config: Optional[Dict[str, Any]] = None