feat(ui): 底栏可拖动切换 + 透明度降至 0.24

- 滑块随手指实时移动，松手后跳转到目标 tab
- 抽出 tabBarGeometry 计算拖拽命中，配套单测
- 底栏浅色/深色底色 0.32 -> 0.24

tools/liquid-glass.test.mjs

@@ -15,8 +15,8 @@ test('custom tab bar renders the liquid-glass overlay layers', () => {
 
 test('custom tab bar uses bright glass material and the silver rim', () => {
   const css = read('src/custom-tab-bar/index.scss')
-  assert.match(css, /background: rgba\(255, 255, 255, 0\.32\)/)
-  assert.match(css, /background: rgba\(25, 25, 25, 0\.32\)/)
+  assert.match(css, /background: rgba\(255, 255, 255, 0\.24\)/)
+  assert.match(css, /background: rgba\(25, 25, 25, 0\.24\)/)
   assert.match(css, /backdrop-filter: blur\(32px\) saturate\(1\.7\) brightness\(1\.05\)/)
   assert.match(css, /-webkit-backdrop-filter: blur\(32px\) saturate\(1\.7\) brightness\(1\.05\)/)
   assert.match(css, /mask-composite: exclude/)