Add display aspect ratio correction for anamorphic DVD previews

Compute DAR from SAR metadata (e.g. 32:27 for anamorphic DVDs) and
pass it to the before/after comparison widget, which uses FittedBox
to stretch the preview to the correct display aspect ratio.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: StuartCameronCode

app/lib/views/preview_panel.dart

@@ -42,13 +42,31 @@ class _PreviewPanelState extends State<PreviewPanel> {
   }
 
   Widget _buildPreviewComparison(BuildContext context, MainViewModel viewModel) {
+    // Compute display aspect ratio for SAR correction.
+    // DAR = (width / height) * (sarNum / sarDen)
+    // Only needed when SAR is non-square (e.g. anamorphic DVD 32:27).
+    double? displayAspectRatio;
+    final videoInfo = viewModel.videoInfo;
+    if (videoInfo != null && videoInfo.sar != null) {
+      final parts = videoInfo.sar!.split(':');
+      if (parts.length == 2) {
+        final sarNum = double.tryParse(parts[0]);
+        final sarDen = double.tryParse(parts[1]);
+        if (sarNum != null && sarDen != null && sarDen > 0) {
+          displayAspectRatio =
+              (videoInfo.width / videoInfo.height) * (sarNum / sarDen);
+        }
+      }
+    }
+
     return Padding(
       padding: const EdgeInsets.all(16),
       child: BeforeAfterComparisonWidget(
         beforeImage: viewModel.currentFrame,
         afterImage: viewModel.processedPreview ?? viewModel.currentFrame,
         isBeforeLoading: viewModel.isAnalyzing,
         isAfterLoading: viewModel.isGeneratingPreview,
+        displayAspectRatio: displayAspectRatio,
       ),
     );
   }

app/lib/widgets/before_after_comparison.dart

@@ -28,13 +28,18 @@ class BeforeAfterComparisonWidget extends StatefulWidget {
   /// Whether the after image is loading.
   final bool isAfterLoading;
 
+  /// Display aspect ratio for SAR correction (e.g. 16/9 for anamorphic DVD).
+  /// When null, the image's native pixel aspect ratio is used (square pixels).
+  final double? displayAspectRatio;
+
   const BeforeAfterComparisonWidget({
     super.key,
     this.beforeImage,
     this.afterImage,
     this.initialDividerPosition = 0.5,
     this.isBeforeLoading = false,
     this.isAfterLoading = false,
+    this.displayAspectRatio,
   });
 
   @override
@@ -412,6 +417,28 @@ class _BeforeAfterComparisonWidgetState
     required String label,
   }) {
     if (imageData != null) {
+      // For non-square pixels (anamorphic DVD etc.), the image's pixel
+      // dimensions don't match the intended display aspect ratio.
+      // FittedBox scales its child uniformly to fit the parent while
+      // preserving the SizedBox's aspect ratio — works even under the
+      // tight constraints from StackFit.expand.
+      if (widget.displayAspectRatio != null) {
+        return FittedBox(
+          fit: BoxFit.contain,
+          child: SizedBox(
+            width: widget.displayAspectRatio! * 1000,
+            height: 1000,
+            child: Image.memory(
+              imageData,
+              fit: BoxFit.fill,
+              gaplessPlayback: true,
+              filterQuality: FilterQuality.none,
+            ),
+          ),
+        );
+      }
+
+      // Square pixels: contain maintains the image's native aspect ratio.
       return Image.memory(
         imageData,
         fit: BoxFit.contain,