fix(dark): 调亮暗黑色调 + dock 跟随暗黑切换 + 图标文字放大

- theme/app.wxss: 暗黑背景 #0F0F12->#2C2C2E,卡片 #1C1C1E->#3A3A3C,
  bg-soft/border 同步提亮,拉开卡片与背景对比(原太黑看不到卡片)
- custom-tab-bar: dock 注册 wx.onThemeChange,系统切暗黑/明亮实时跟随
  (组件不在 getCurrentPages,app.js watchDarkMode 通知不到)
- custom-tab-bar: 新增 updateTheme,settings 手动切深色时通知 dock
- custom-tab-bar.wxss: dock 图标 44->50rpx,文字 20->23rpx,未激活
  opacity 0.5->0.7(更醒目)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

custom-tab-bar/index.wxss

@@ -36,18 +36,18 @@
 }
 
 .dock-icon {
-  width: 44rpx;
-  height: 44rpx;
+  width: 50rpx;
+  height: 50rpx;
   background-size: contain;
   background-repeat: no-repeat;
   background-position: center;
-  margin-bottom: 4rpx;
-  opacity: 0.5;
+  margin-bottom: 5rpx;
+  opacity: 0.7;
   transition: opacity 0.2s ease;
 }
 
 .dock-text {
-  font-size: 20rpx;
+  font-size: 23rpx;
   color: var(--text-secondary, #999);
   line-height: 1;
   transition: color 0.2s ease;