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

src/custom-tab-bar/index.tsx

@@ -1,10 +1,11 @@
 import { View, Text, Image } from '@tarojs/components'
 import Taro from '@tarojs/taro'
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import { getTheme, resolveTheme, setDetectedSystemTheme } from '../utils/store'
 import { THEME_CHANGE_EVENT } from '../context/ThemeContext'
 import { applyPageBackground } from '../utils/themeBackground'
 import { assetUrl } from '../utils/asset'
+import { clampDragX, getTabBarFrame, tabIndexAtX } from './tabBarGeometry'
 import './index.scss'
 
 const TABS = [
@@ -15,6 +16,8 @@ const TABS = [
   { pagePath: '/pages/profile/index', label: '我的', icon: assetUrl('/icon/个人.svg'), iconActive: assetUrl('/icon/个人-fill.svg') }
 ]
 
+const TAB_CENTER_STEP = 100 / TABS.length
+
 function matchTab(path: string): string {
   const idx = TABS.findIndex((t) => path.endsWith(t.pagePath))
   return TABS[idx >= 0 ? idx : 0].pagePath
@@ -36,6 +39,9 @@ export default function CustomTabBar() {
 
   // 微信自定义 tabBar 不会自动感知 page 路由变化，用 state 保持选中态响应式
   const [activeTab, setActiveTab] = useState<string>(currentTabPath)
+  const [dragX, setDragX] = useState<number | null>(null)
+  const dragXRef = useRef<number | null>(null)
+  const barFrameRef = useRef<{ left: number; width: number } | null>(null)
 
   useEffect(() => {
     const handler = (t: 'light' | 'dark') => setResolvedTheme(t)
@@ -91,21 +97,68 @@ export default function CustomTabBar() {
     Taro.eventCenter.trigger('tabBarChange', url)
   }
 
+  const startDrag = (e: any) => {
+    const touch = e.touches && e.touches[0]
+    if (!touch) return
+    const frame = getTabBarFrame(Taro.getSystemInfoSync().windowWidth)
+    barFrameRef.current = frame
+    const nextX = clampDragX(touch.clientX - frame.left, frame.width)
+    dragXRef.current = nextX
+    setDragX(nextX)
+  }
+
+  const moveDrag = (e: any) => {
+    if (dragXRef.current === null) return
+    const frame = barFrameRef.current
+    const touch = e.touches && e.touches[0]
+    if (!frame || !touch) return
+    const nextX = clampDragX(touch.clientX - frame.left, frame.width)
+    dragXRef.current = nextX
+    setDragX(nextX)
+  }
+
+  const endDrag = () => {
+    const frame = barFrameRef.current
+    const x = dragXRef.current
+    if (frame && x !== null) {
+      const idx = tabIndexAtX(x, frame.width, TABS.length)
+      const target = TABS[idx]
+      if (target && target.pagePath !== activeTab) switchTab(target.pagePath)
+    }
+    barFrameRef.current = null
+    dragXRef.current = null
+    setDragX(null)
+  }
+
+  const cancelDrag = () => {
+    barFrameRef.current = null
+    dragXRef.current = null
+    setDragX(null)
+  }
+
+  const activeIndex = Math.max(0, TABS.findIndex((t) => t.pagePath === activeTab))
+
   return (
     <View className={`tab-bar-root theme-${resolvedTheme}`}>
       <View className='tab-bar-gradient' />
-      <View className={`custom-tab-bar theme-${resolvedTheme}`}>
+      <View
+        className={`custom-tab-bar theme-${resolvedTheme}`}
+        onTouchStart={startDrag}
+        onTouchMove={moveDrag}
+        onTouchEnd={endDrag}
+        onTouchCancel={cancelDrag}
+        catchMove
+      >
         <View className='tab-bar-caustic' />
         <View className='tab-bar-rim' />
+        <View
+          className={`tab-indicator ${dragX !== null ? 'dragging' : ''}`}
+          style={{ left: dragX === null ? `calc(${(activeIndex + 0.5) * TAB_CENTER_STEP}%)` : `${dragX}px` }}
+        />
         {TABS.map((tab) => {
           const isActive = tab.pagePath === activeTab
           return (
-            <View
-              key={tab.pagePath}
-              className={`tab-item ${isActive ? 'active' : ''}`}
-              onTap={() => switchTab(tab.pagePath)}
-            >
-              {isActive && <View className='tab-indicator' />}
+            <View key={tab.pagePath} className={`tab-item ${isActive ? 'active' : ''}`}>
               <Image className={`tab-icon-img ${isActive ? 'active' : ''}`} src={isActive ? tab.iconActive : tab.icon} mode='aspectFit' />
               <Text className='tab-label'>{tab.label}</Text>
             </View>