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/lib/pages/capture_page.dart

@@ -139,9 +139,6 @@ class CameraPluginSource extends CaptureSource {
     _zoomLevel = clamped;
   }
 
-  /// 当前变焦倍数（最近一次硬件同步值）。
-  double get currentZoom => _zoomLevel;
-
   @override
   bool get supportsFlash => true;
 
@@ -377,7 +374,10 @@ class _CapturePageState extends ConsumerState<CapturePage>
     _pinchBaseZoom = _lastSentZoom
         .clamp(source.minZoom, source.maxZoom)
         .toDouble();
-    _showZoomBadge(_pinchBaseZoom!);
+    // 单指轻点同样触发 onScaleStart——倍数徽标仅在真捏合（≥2 指）时显示，
+    // 避免轻点预览也弹 1.0×；基线重定保持无条件（单指转捏合、抬起再捏合
+    // 均从当前倍数续接）。
+    if (details.pointerCount >= 2) _showZoomBadge(_pinchBaseZoom!);
   }
 
   void _onPinchUpdate(ScaleUpdateDetails details) {