fix(app-store): Update update count in navigation

[susnux]

• Resolves: When updating apps, the number of apps to update (left side panel) doesn't update (you have to refresh the page). #48862

Summary

The update count was not reactive anymore since the work on the discover feature.
So this is only bandage, but the proper fix of replacing the vuex store fully with the second pinia store is only possible or 31.

Screen recording

Bildschirmaufnahme_20241023_192659.webm

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable30

[susnux]

/backport to stable29