fix(import): address PR review concerns

- remove duplicate List import in AdminControllerTest
- derive skipped() from skippedFiles.size() — drop redundant int field
- use machine codes for SkippedFile.reason (INVALID_PDF_SIGNATURE, FILE_READ_ERROR)
- map reason codes to i18n strings in ImportStatusCard (de/en/es)
- replace raw amber Tailwind classes with warning semantic token
- fix <summary> accessibility: replace list-none with rotating chevron SVG
- replace <p> with <span> inside <summary> (phrasing content rule)
- extract setupOneValidOneFakeImport() helper — remove 3x copy-paste
- add lenient mock to short-file test for defensive coverage
- add IOException path test for isPdfMagicBytes

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

frontend/src/routes/admin/system/ImportStatusCard.svelte.test.ts

@@ -140,9 +140,9 @@ describe('ImportStatusCard', () => {
 					processed: 10,
 					skipped: 3,
 					skippedFiles: [
-						{ filename: 'fake.pdf', reason: 'Keine gültige PDF-Signatur' },
-						{ filename: 'other.pdf', reason: 'Keine gültige PDF-Signatur' },
-						{ filename: 'tiny.pdf', reason: 'Keine gültige PDF-Signatur' }
+						{ filename: 'fake.pdf', reason: 'INVALID_PDF_SIGNATURE' },
+						{ filename: 'other.pdf', reason: 'INVALID_PDF_SIGNATURE' },
+						{ filename: 'tiny.pdf', reason: 'INVALID_PDF_SIGNATURE' }
 					]
 				}),
 				ontrigger: () => {}
@@ -160,7 +160,7 @@ describe('ImportStatusCard', () => {
 					statusCode: 'IMPORT_DONE',
 					processed: 5,
 					skipped: 1,
-					skippedFiles: [{ filename: 'fake.pdf', reason: 'Keine gültige PDF-Signatur' }]
+					skippedFiles: [{ filename: 'fake.pdf', reason: 'INVALID_PDF_SIGNATURE' }]
 				}),
 				ontrigger: () => {}
 			}