fix(ui): 移除快速切换时闪烁的整页淡入动画

- 保留首帧前切好主题背景
- 去掉页面根容器从透明度 0 重新淡入的动画，避免快速点 tab 时闪系统底色

src/app.scss

@@ -10,19 +10,12 @@
   min-height: 100vh;
   background-color: var(--bg-page, #ffffff);
   box-sizing: border-box;
-  animation: page-fade-in 0.24s ease both;
 }
 
 .theme-dark {
   min-height: 100vh;
   background-color: var(--bg-page, #191919);
   box-sizing: border-box;
-  animation: page-fade-in 0.24s ease both;
-}
-
-@keyframes page-fade-in {
-  from { opacity: 0; }
-  to { opacity: 1; }
 }
 
 /* 变量定义 */

tools/liquid-glass.test.mjs

@@ -36,10 +36,9 @@ test('scroll top back button matches the global liquid-glass back style', () =>
   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', () => {
+test('app sets page background before first render without a blinking full fade', () => {
   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/)
+  assert.doesNotMatch(appCss, /animation:\s*page-fade-in/)
 })