feat: 后端接线上 React 构建产物，legacy 模板降级为兜底

为什么：前端迁移到 Vite 后，首页应直接返回 frontend/dist/index.html
字节（golden 口径随之变为构建产物原样字节），仅当 dist 未构建时
回退 Jinja2 渲染的 legacy 模板。同步挂载 /assets 静态目录、排除
frontend 的 node_modules/tsbuildinfo 入库、docker 构建只带 dist
不带源码、ruff/pyright 排除 frontend 目录（TS 侧由 tsc+vite 把关）。

pyproject.toml

@@ -38,7 +38,7 @@ package = false
 [tool.ruff]
 target-version = "py312"
 line-length = 120
-extend-exclude = [".venv", ".zcode", "templates", "static", "data"]
+extend-exclude = [".venv", ".zcode", "templates", "static", "frontend", "data"]
 
 [tool.ruff.lint]
 # E/F 基础错误、I 导入排序、B bugbear、UP 升级、C4 推导式、ASYNC 异步陷阱、S 安全、RUF
@@ -90,7 +90,7 @@ include = [
     "music_downloader",
     "_mkpkg.py",
 ]
-exclude = [".venv", ".zcode", "data", "downloads", "templates", "static"]
+exclude = [".venv", ".zcode", "data", "downloads", "templates", "static", "frontend"]
 venvPath = "."
 venv = ".venv"
 pythonVersion = "3.12"