feat(auth): frontend ErrorCode + i18n for INVALID_CREDENTIALS and SESSION_EXPIRED

Mirrors the backend ErrorCode additions from commit 393a3c25.
Adds error_session_expired_explainer for the login-page banner that
will surface when ?reason=expired.

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

frontend/src/lib/shared/errors.ts

@@ -44,6 +44,8 @@ export type ErrorCode =
 	| 'CIRCULAR_RELATIONSHIP'
 	| 'DUPLICATE_RELATIONSHIP'
 	| 'GESCHICHTE_NOT_FOUND'
+	| 'INVALID_CREDENTIALS'
+	| 'SESSION_EXPIRED'
 	| 'MISSING_CREDENTIALS'
 	| 'UNAUTHORIZED'
 	| 'FORBIDDEN'
@@ -154,6 +156,10 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_duplicate_relationship();
 		case 'GESCHICHTE_NOT_FOUND':
 			return m.error_geschichte_not_found();
+		case 'INVALID_CREDENTIALS':
+			return m.error_invalid_credentials();
+		case 'SESSION_EXPIRED':
+			return m.error_session_expired();
 		case 'MISSING_CREDENTIALS':
 			return m.login_error_missing_credentials();
 		case 'UNAUTHORIZED':