fix(ui): 首屏背景提前设置 + 页面淡入过渡

- App 首帧前同步主题窗口背景，减少深浅色下系统底色闪烁
- 页面根容器增加 0.24s 淡入动画，Tab 切换/进入更平滑

tools/liquid-glass.test.mjs

@@ -35,3 +35,11 @@ test('scroll top back button matches the global liquid-glass back style', () =>
   assert.match(css, /background: linear-gradient\(\s*135deg/)
   assert.match(css, /backdrop-filter: blur\(24px\) saturate\(1\.7\) brightness\(1\.05\)/)
 })
+
+test('app sets page background before first render and fades pages in', () => {
+  const appSource = read('src/app.tsx')
+  const appCss = read('src/app.scss')
+  assert.match(appSource, /componentWillMount/)
+  assert.match(appCss, /@keyframes page-fade-in/)
+  assert.match(appCss, /animation: page-fade-in/)
+})