feat: Add unread badge to sidebar for inbox view (#10803)

# Pull Request Template

## Description

This PR adds a badge to the icon in the sidebar group header and an
unread highlight for the inbox view.

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

**Loom video**

https://www.loom.com/share/1a880aa16de249a1836b8d014325c067?sid=87cc7b64-667b-4066-8993-d33abccf7b67


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/components-next/sidebar/Sidebar.vue

@@ -151,6 +151,9 @@ const menuItems = computed(() => {
       icon: 'i-lucide-inbox',
       to: accountScopedRoute('inbox_view'),
       activeOn: ['inbox_view', 'inbox_view_conversation'],
+      getterKeys: {
+        badge: 'notifications/getHasUnreadNotifications',
+      },
     },
     {
       name: 'Conversation',