feat(api): 支持挂载 Nuxt 静态 UI 并统一桌面端运行参数

- 若存在 WECHAT_TOOL_UI_DIR 或 frontend/.output/public，则在 / 挂载静态站点并支持 SPA 路由回退
- API 根端点调整为 /api，避免与静态 UI 冲突
- 新增 WECHAT_TOOL_DATA_DIR 输出目录约定，统一 databases/key store 等路径
- host/port 支持通过 WECHAT_TOOL_HOST / WECHAT_TOOL_PORT 配置，并打印可访问地址
- 新增 backend_entry.py 作为 PyInstaller 入口，减少动态导入识别问题

src/wechat_decrypt_tool/backend_entry.py

@@ -0,0 +1,21 @@
+"""Entry point for bundling the FastAPI backend into a standalone executable.
+
+This avoids dynamic import strings like "pkg.module:app" which some bundlers
+cannot detect reliably.
+"""
+
+import os
+
+import uvicorn
+
+from wechat_decrypt_tool.api import app
+
+
+def main() -> None:
+    host = os.environ.get("WECHAT_TOOL_HOST", "127.0.0.1")
+    port = int(os.environ.get("WECHAT_TOOL_PORT", "8000"))
+    uvicorn.run(app, host=host, port=port, log_level="info")
+
+
+if __name__ == "__main__":
+    main()