fix(ui): 初始背景跟随外观 + 修复跟随系统

- app.config 移除写死的浅色背景，交给 theme.json 按系统外观选择首帧底色
- 跟随系统模式增加 setInterval 轮询兜底，onThemeChange 不回调时也能跟上切换

tools/liquid-glass.test.mjs

@@ -42,3 +42,16 @@ test('app sets page background before first render without a blinking full fade'
   assert.match(appSource, /componentWillMount/)
   assert.doesNotMatch(appCss, /animation:\s*page-fade-in/)
 })
+
+test('delegates initial background to themeLocation instead of hardcoding light', () => {
+  const config = read('src/app.config.ts')
+  assert.match(config, /themeLocation:\s*'theme\.json'/)
+  assert.doesNotMatch(config, /navigationBarBackgroundColor:\s*'#faf7f2'/)
+  assert.doesNotMatch(config, /backgroundColorTop:\s*'#faf7f2'/)
+})
+
+test('polls the system theme while following the system', () => {
+  const ctx = read('src/context/ThemeContext.tsx')
+  assert.match(ctx, /setInterval/)
+  assert.match(ctx, /getAppBaseInfo\(\)/)
+})