feat(helpdesk): add empty Dashboard page with sidebar nav item

Adds helpdesk/dashboard route, empty page/view, Overview nav group in the
sidebar, and Dashboard i18n keys (de + en). No content yet.

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

modules/helpdesk/i18n/default_en.json

@@ -37,6 +37,7 @@
   "No tickets found": "No tickets found",
   "No tickets are linked to this debtor in Business Central.": "No tickets are linked to this debtor in Business Central.",
   "Ticket No.": "Ticket No.",
+  "Dashboard": "Dashboard",
   "Description": "Description",
   "Status": "Status",
   "Category": "Category",