feat(app): camera preview WYSIWYG aspect + pinch zoom with double-tap reset

Preview now centers at the sensor aspect ratio (letterboxed on the dark
backdrop) instead of FittedBox cover, which cropped ~38% horizontally and
made the preview differ from the full-frame takePicture output.

Zoom capability hangs off CaptureSource as default implementations
(minZoom/maxZoom = 1, setZoom no-op) so existing fakes stay untouched;
CameraPluginSource probes getMin/MaxZoomLevel after initialize (failure
keeps 1/1 and disables the gesture). The page attaches a scale
GestureDetector only when maxZoom > 1: pinch drives setZoom (throttled at
0.05 delta, serialized to prevent out-of-order platform calls), a badge
shows the current level and hides after 1s, double-tap resets to 1.0x,
and camera flip/reopen resets zoom state. _StaticCaptureSource in
album_detail_page_test.dart switches implements -> extends to inherit the
new defaults.

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

apps/mobile/test/pages/album_detail_page_test.dart

@@ -403,8 +403,9 @@ void main() {
   });
 }
 
-/// 相机源替身（详情页跳连拍页测试：仅就绪，不拍摄）。
-class _StaticCaptureSource implements CaptureSource {
+/// 相机源替身（详情页跳连拍页测试：仅就绪，不拍摄）。extends 继承
+/// 接口默认实现（闪光灯/变焦能力默认不支持——UI 不显示开关不启用手势）。
+class _StaticCaptureSource extends CaptureSource {
   @override
   Future<void> initialize() async {}
 
@@ -414,12 +415,6 @@ class _StaticCaptureSource implements CaptureSource {
   @override
   Widget buildPreview() => const SizedBox();
 
-  @override
-  bool get supportsFlash => false;
-
-  @override
-  Future<void> setFlashEnabled(bool enabled) async {}
-
   @override
   Future<void> dispose() async {}
 }