refactor: 将 WebUI 源码纳入仓库,移除打包产物

- 新增 webui/ 前端源码目录（React+Vite+TypeScript+Tailwind）
- 删除 api/webui/ 中旧的打包静态资源
- 更新 .gitignore 忽略 api/webui/ 构建输出和 webui/node_modules
- 更新 README 增加前端构建说明

webui/src/store/themeStore.ts

@@ -0,0 +1,69 @@
+import { create } from 'zustand'
+
+type Theme = 'light' | 'dark' | 'system'
+
+interface ThemeState {
+  theme: Theme
+  resolvedTheme: 'light' | 'dark'
+  setTheme: (theme: Theme) => void
+}
+
+const THEME_KEY = 'mediacrawler_theme'
+
+function getSystemTheme(): 'light' | 'dark' {
+  if (typeof window === 'undefined') return 'light'
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+}
+
+function getStoredTheme(): Theme {
+  if (typeof window === 'undefined') return 'light'
+  const stored = localStorage.getItem(THEME_KEY) as Theme | null
+  if (stored && ['light', 'dark', 'system'].includes(stored)) {
+    return stored
+  }
+  return 'light' // Default to light
+}
+
+function applyTheme(resolved: 'light' | 'dark') {
+  const root = document.documentElement
+  if (resolved === 'dark') {
+    root.classList.add('dark')
+  } else {
+    root.classList.remove('dark')
+  }
+}
+
+function resolveTheme(theme: Theme): 'light' | 'dark' {
+  return theme === 'system' ? getSystemTheme() : theme
+}
+
+// Initialize theme immediately to prevent flash
+const initialTheme = getStoredTheme()
+const initialResolved = resolveTheme(initialTheme)
+if (typeof window !== 'undefined') {
+  applyTheme(initialResolved)
+}
+
+export const useThemeStore = create<ThemeState>((set) => ({
+  theme: initialTheme,
+  resolvedTheme: initialResolved,
+
+  setTheme: (theme) => {
+    const resolved = resolveTheme(theme)
+    localStorage.setItem(THEME_KEY, theme)
+    applyTheme(resolved)
+    set({ theme, resolvedTheme: resolved })
+  },
+}))
+
+// Listen for system theme changes
+if (typeof window !== 'undefined') {
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', (e) => {
+    const state = useThemeStore.getState()
+    if (state.theme === 'system') {
+      const resolved = e.matches ? 'dark' : 'light'
+      applyTheme(resolved)
+      useThemeStore.setState({ resolvedTheme: resolved })
+    }
+  })
+}