fix: count checksum-corrupted media as missing on local backup import

Import result missingMediaCount compared restored assets against the
backup's declared media file list, so an embedded file that failed
sha256 verification was still treated as present. Compute it from the
actually restored files instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/application/use_cases.dart

@@ -208,7 +208,12 @@ final class DataImportUseCase {
         mode: mode,
         importedAt: clock.now(),
       );
-      return result.copyWith(missingMediaCount: _missingMediaCount(snapshot));
+      return result.copyWith(
+        missingMediaCount: _missingRestoredMediaCount(
+          decoded,
+          restoredMediaFiles,
+        ),
+      );
     } on LocalBackupException {
       rethrow;
     } on Object {
@@ -4993,6 +4998,15 @@ int _missingMediaCount(LocalDataExportSnapshot snapshot) {
       .length;
 }
 
+int _missingRestoredMediaCount(
+  LocalDataExportSnapshot snapshot,
+  Map<String, RestoredMediaFile> restoredFiles,
+) {
+  return snapshot.mediaAssets
+      .where((asset) => !restoredFiles.containsKey(asset.id))
+      .length;
+}
+
 bool _hasNameDuplicates(LocalDataExportSnapshot snapshot) {
   return _hasDuplicatesByName(snapshot.exercises) ||
       _hasDuplicatesByName(snapshot.programs) ||