feat(comments): add CommentThread, annotation panel, Diskussion section, and i18n keys

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

frontend/src/lib/errors.ts

@@ -16,6 +16,7 @@ export type ErrorCode =
 	| 'INVALID_RESET_TOKEN'
 	| 'ANNOTATION_NOT_FOUND'
 	| 'ANNOTATION_OVERLAP'
+	| 'COMMENT_NOT_FOUND'
 	| 'UNAUTHORIZED'
 	| 'FORBIDDEN'
 	| 'VALIDATION_ERROR'
@@ -67,6 +68,8 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_annotation_not_found();
 		case 'ANNOTATION_OVERLAP':
 			return m.error_annotation_overlap();
+		case 'COMMENT_NOT_FOUND':
+			return m.error_comment_not_found();
 		case 'UNAUTHORIZED':
 			return m.error_unauthorized();
 		case 'FORBIDDEN':