feat(frontend): invite-based registration UI

- Add /register route with invite code prefill, password show/hide
- Add /login?registered=1 success banner
- Add /admin/invites page: list, create, revoke, copy link
- Add Einladungen nav section to admin sidebar (ADMIN_USER perm)
- Add invite error codes to errors.ts
- Add 48 i18n keys across de/en/es
- Update hooks.server.ts to allow public access to invite/register API

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

frontend/src/lib/errors.ts

@@ -17,6 +17,10 @@ export type ErrorCode =
 	| 'WRONG_CURRENT_PASSWORD'
 	| 'IMPORT_ALREADY_RUNNING'
 	| 'INVALID_RESET_TOKEN'
+	| 'INVITE_NOT_FOUND'
+	| 'INVITE_EXHAUSTED'
+	| 'INVITE_REVOKED'
+	| 'INVITE_EXPIRED'
 	| 'ANNOTATION_NOT_FOUND'
 	| 'ANNOTATION_UPDATE_FAILED'
 	| 'TRANSCRIPTION_BLOCK_NOT_FOUND'
@@ -87,6 +91,14 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_import_already_running();
 		case 'INVALID_RESET_TOKEN':
 			return m.error_invalid_reset_token();
+		case 'INVITE_NOT_FOUND':
+			return m.error_invite_not_found();
+		case 'INVITE_EXHAUSTED':
+			return m.error_invite_exhausted();
+		case 'INVITE_REVOKED':
+			return m.error_invite_revoked();
+		case 'INVITE_EXPIRED':
+			return m.error_invite_expired();
 		case 'ANNOTATION_NOT_FOUND':
 			return m.error_annotation_not_found();
 		case 'ANNOTATION_UPDATE_FAILED':