Add seenByTenant tracking feature

- Add seenByTenant field to BillingLocation interface
- Implement setSeenByTenant function to mark locations as viewed by tenant
  - Checks if flag is already set to avoid unnecessary DB updates
  - Includes TypeDoc documentation
- Update LocationViewPage to call setSeenByTenant when non-owner visits
- Add seenByTenant to fetchAllLocations projection
- Update LocationCard to show "seen by tenant" status indicator
  - Displays in "Monthly statement" fieldset with checkmark icon
  - Shows alongside monthly expense total
- Add localization strings for monthly statement and seen status

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

messages/en.json

@@ -55,7 +55,9 @@
       "edit-card-tooltip": "Edit realestate",
       "add-bill-button-tooltip": "Add a new bill",
       "payed-total-label": "Payed total:",
-      "link-copy-message": "Link copied to clipboard"
+      "link-copy-message": "Link copied to clipboard",
+      "monthly-statement-legend": "Monthly statement",
+      "seen-by-tenant-label": "Seen by tenant"
     },
     "month-card": {
       "payed-total-label": "Total monthly expenditure:",