Refactor: Add null check and type guard for window.fejson access

[strawmelonjuice]

The function could benefit from stronger type safety when accessing window.fejson.

 export function getJsonObj() {
-	if (!window.fejson) {
+	if (!window.fejson || !isFejsonObject(window.fejson)) {
 		return {
 			instance_iid: -1,
 			instance_lastsync: -1,
 			pulled: 0,
 			user_id: -1,
 			user_username: "unset",
 			user_email: "unset",
 		};
 	}
 	return {
 		instance_iid: window.fejson.instance.iid,
 		instance_lastsync: window.fejson.instance.lastsync,
 		pulled: window.fejson.pulled,
 		user_id: window.fejson.user.id,
 		user_username: window.fejson.user.username,
 		user_email: window.fejson.user.email,
 	};
 }

+function isFejsonObject(obj: any): obj is fejsonObject {
+    return obj &&
+        typeof obj.pulled === 'number' &&
+        obj.instance &&
+        typeof obj.instance.iid === 'string' &&
+        typeof obj.instance.lastsync === 'number' &&
+        obj.user &&
+        typeof obj.user.username === 'string' &&
+        typeof obj.user.id === 'number' &&
+        typeof obj.user.email === 'string';
+}

Committable suggestion skipped: line range outside the PR's diff.

Originally posted by @coderabbitai[bot] in #47 (comment)