fix(import): address round-3 review concerns

- Add comment to openFileStream() explaining package-private visibility
  is intentional (Mockito spy seam for IOException test)
- Key {#each} skippedFiles by filename instead of array index
- Add test: skipped section hidden when state is FAILED
- Add test: reasonLabel returns raw code for unknown reason strings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java

@@ -312,6 +312,7 @@ public class MassImportService {
         return new ProcessResult(processed, skippedFiles);
     }
 
+    // package-private: Mockito spy in tests can override to inject IOException
     InputStream openFileStream(File file) throws IOException {
         return new FileInputStream(file);
     }