fix(inbox): size indicator not updating on item deletion (@byseif21) (monkeytypegame#6649)

### Description

Closes monkeytypegame#6648

Author: byseif21

frontend/src/ts/db.ts

@@ -993,6 +993,14 @@ export function addXp(xp: number): void {
   setSnapshot(snapshot);
 }
 
+export function updateInboxUnreadSize(newSize: number): void {
+  const snapshot = getSnapshot();
+  if (!snapshot) return;
+
+  snapshot.inboxUnreadSize = newSize;
+  setSnapshot(snapshot);
+}
+
 export function addBadge(badge: Badge): void {
   const snapshot = getSnapshot();
   if (!snapshot) return;

frontend/src/ts/elements/alerts.ts

@@ -29,6 +29,7 @@ const state: State = {
 
 function hide(): void {
   setNotificationBubbleVisible(false);
+  DB.updateInboxUnreadSize(0);
   void modal.hide({
     afterAnimation: async () => {
       $("#alertsPopup .notificationHistory .list").empty();
@@ -301,8 +302,9 @@ export function setNotificationBubbleVisible(tf: boolean): void {
 }
 
 function updateInboxSize(): void {
+  const remainingItems = accountAlerts.length - mailToDelete.length;
   $("#alertsPopup .accountAlerts .title .right").text(
-    `${accountAlerts.length}/${maxMail}`
+    `${remainingItems}/${maxMail}`
   );
 }