feat(#221): add i18n keys and error codes for tag hierarchy errors

Adds INVALID_TAG_COLOR and TAG_CYCLE_DETECTED to the frontend ErrorCode
type and getErrorMessage() switch. German, English, and Spanish
translations added for both codes.

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

frontend/src/lib/errors.ts

@@ -27,6 +27,8 @@ export type ErrorCode =
 	| 'OCR_DOCUMENT_NOT_UPLOADED'
 	| 'OCR_PROCESSING_FAILED'
 	| 'TRAINING_ALREADY_RUNNING'
+	| 'INVALID_TAG_COLOR'
+	| 'TAG_CYCLE_DETECTED'
 	| 'UNAUTHORIZED'
 	| 'FORBIDDEN'
 	| 'VALIDATION_ERROR'
@@ -100,6 +102,10 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_ocr_processing_failed();
 		case 'TRAINING_ALREADY_RUNNING':
 			return m.error_training_already_running();
+		case 'INVALID_TAG_COLOR':
+			return m.error_invalid_tag_color();
+		case 'TAG_CYCLE_DETECTED':
+			return m.error_tag_cycle_detected();
 		case 'UNAUTHORIZED':
 			return m.error_unauthorized();
 		case 'FORBIDDEN':