fix(app): lock capture page portrait; gate zoom badge on 2 pointers; drop dead getter

- MainActivity locked to portrait: the capture page top bar / bottom
  controls and the preview aspect swap all assume portrait; landscape
  misaligns the preview ratio and control layers.
- Zoom badge now requires details.pointerCount >= 2 on onScaleStart so a
  single stationary tap no longer pops a 1.0x badge for 1s; base-zoom
  rebasing stays unconditional (single-finger-into-pinch still resumes
  from the current level).
- Removed unused CameraPluginSource.currentZoom getter.

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

apps/mobile/android/app/src/main/AndroidManifest.xml

@@ -30,11 +30,14 @@
         <meta-data
             android:name="io.flutter.embedding.android.EnableImpeller"
             android:value="false" />
+        <!-- 连拍页按竖屏设计（顶部常驻层/底部控制位与预览宽高交换均假设
+             纵向），横屏下预览比例与控制层错位；锁定竖屏。 -->
         <activity
             android:name=".MainActivity"
             android:exported="true"
             android:launchMode="singleTop"
             android:taskAffinity=""
+            android:screenOrientation="portrait"
             android:theme="@style/LaunchTheme"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
             android:hardwareAccelerated="true"