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/capture_page_test.dart

@@ -5,7 +5,8 @@
 /// 准备器 + 非 Android 权限门替身。覆盖：快门即拍即入队、连拍串行、分类
 /// 切换仅影响之后拍摄、多选补充入队、待传/失败徽章实时刷新、有待传任务
 /// 退出确认、权限拒绝/初始化失败视图与重试、深色界面（#10161E）、快门
-/// ≥72dp、闪光灯按能力显示、前后摄切换重建、最近一张缩略图。
+/// ≥72dp、闪光灯按能力显示、前后摄切换重建、最近一张缩略图、捏合变焦
+/// 与双击复位。
 library;
 
 import 'dart:io';
@@ -81,6 +82,23 @@ class FakeCaptureSource extends CaptureSource {
   Future<void> dispose() async => disposeCalls++;
 }
 
+/// 变焦能力替身（Task 7 捏合变焦）：能力 1-8×，记录 setZoom 调用序列。
+/// 继承 [FakeCaptureSource]（预览标记/初始化/释放行为一致）。
+class ZoomFakeCaptureSource extends FakeCaptureSource {
+  ZoomFakeCaptureSource(super.files);
+
+  final List<double> zoomCalls = <double>[];
+
+  @override
+  double get minZoom => 1;
+
+  @override
+  double get maxZoom => 8;
+
+  @override
+  Future<void> setZoom(double level) async => zoomCalls.add(level);
+}
+
 void main() {
   late FakeSessionStore session;
   late TaskStore store;
@@ -147,7 +165,8 @@ void main() {
         categories: categories,
       );
 
-  Future<void> pumpCapture(WidgetTester tester) async {
+  Future<void> pumpCapture(WidgetTester tester, {CaptureSource? cameraSource}) async {
+    final effectiveSource = cameraSource ?? source;
     await tester.pumpWidget(
       ProviderScope(
         overrides: [
@@ -164,7 +183,7 @@ void main() {
           captureSourceFactoryProvider.overrideWith(
             (ref) => ({bool preferFront = false}) async {
               factoryPreferFront.add(preferFront);
-              return source;
+              return effectiveSource;
             },
           ),
           uploadPreparerProvider.overrideWith(
@@ -462,4 +481,56 @@ void main() {
     expect(find.text('Ceremony'), findsOneWidget);
     expect(find.text('Gala'), findsOneWidget);
   });
+
+  testWidgets('捏合变焦：双指张开驱动 setZoom 并显示倍数徽标', (tester) async {
+    final zoomSource = ZoomFakeCaptureSource(<File>[]);
+    await pumpCapture(tester, cameraSource: zoomSource);
+    expect(find.text('PREVIEW_MARK'), findsOneWidget, reason: '相机预览就绪');
+
+    final center = tester.getCenter(find.text('PREVIEW_MARK'));
+    // 必须两根指针：ScaleGestureRecognizer 的 scale = 当前指距/初始指距，
+    // 单指针 span=0 → scale 恒为 1.0，永远测不到变焦。
+    final p1 = await tester.startGesture(center.translate(-40, 0));
+    final p2 = await tester.startGesture(center.translate(40, 0));
+    await p1.moveBy(const Offset(-40, 0));
+    await p2.moveBy(const Offset(40, 0));
+    await tester.pump();
+    await p1.up();
+    await p2.up();
+    await tester.pump();
+
+    expect(zoomSource.zoomCalls.last, greaterThan(1.0));
+    expect(find.textContaining('×'), findsOneWidget, reason: '捏合期间显示倍数徽标');
+
+    // 停止 1s 后徽标隐藏；同时冲刷计时器避免悬挂。
+    await tester.pump(const Duration(seconds: 1));
+    expect(find.textContaining('×'), findsNothing, reason: '1s 后徽标隐藏');
+  });
+
+  testWidgets('双击复位 1.0×', (tester) async {
+    final zoomSource = ZoomFakeCaptureSource(<File>[]);
+    await pumpCapture(tester, cameraSource: zoomSource);
+
+    // 先捏合放大到 >1×，再双击复位。
+    final center = tester.getCenter(find.text('PREVIEW_MARK'));
+    final p1 = await tester.startGesture(center.translate(-40, 0));
+    final p2 = await tester.startGesture(center.translate(40, 0));
+    await p1.moveBy(const Offset(-40, 0));
+    await p2.moveBy(const Offset(40, 0));
+    await p1.up();
+    await p2.up();
+    await tester.pump();
+    expect(zoomSource.zoomCalls.last, greaterThan(1.0), reason: '前置：已放大');
+
+    await tester.tap(find.text('PREVIEW_MARK'));
+    await tester.pump(const Duration(milliseconds: 100));
+    await tester.tap(find.text('PREVIEW_MARK'));
+    await tester.pump();
+
+    expect(zoomSource.zoomCalls.last, 1.0);
+    expect(find.text('1.0×'), findsOneWidget, reason: '复位后徽标显示 1.0×');
+
+    // 冲刷徽标隐藏计时器避免悬挂。
+    await tester.pump(const Duration(seconds: 1));
+  });
 }