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>

app/lib/db-types.ts

@@ -63,6 +63,8 @@ export interface BillingLocation {
     rentDueDay?: number | null;
     /** (optional) monthly rent amount in cents */
     rentAmount?: number | null;
+    /** (optional) whether the location has been seen by tenant */
+    seenByTenant?: boolean | null;
 };
 
 export enum BilledTo {