fix(upload): structured error codes for quick-upload, fix duplicate filename crash

- Switch errors from plain strings to { filename, code } objects so the
  frontend can show translated messages instead of raw exception text
- Add UNSUPPORTED_FILE_TYPE error code end-to-end (Java enum → errors.ts
  → de/en/es messages)
- Fix IncorrectResultSizeDataAccessException when a filename exists more
  than once in the DB: use findFirstByOriginalFilename instead of
  findByOriginalFilename in storeDocument()

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

frontend/src/lib/errors.ts

@@ -9,6 +9,7 @@ export type ErrorCode =
 	| 'DOCUMENT_NO_FILE'
 	| 'FILE_NOT_FOUND'
 	| 'FILE_UPLOAD_FAILED'
+	| 'UNSUPPORTED_FILE_TYPE'
 	| 'USER_NOT_FOUND'
 	| 'EMAIL_ALREADY_IN_USE'
 	| 'WRONG_CURRENT_PASSWORD'
@@ -54,6 +55,8 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_file_not_found();
 		case 'FILE_UPLOAD_FAILED':
 			return m.error_file_upload_failed();
+		case 'UNSUPPORTED_FILE_TYPE':
+			return m.error_unsupported_file_type();
 		case 'USER_NOT_FOUND':
 			return m.error_user_not_found();
 		case 'EMAIL_ALREADY_IN_USE':