Add rent amount field to location form

- Add rentAmount field to BillingLocation interface (stored in cents)
- Implement Zod validation with conditional requirement when rent notification is enabled
- Add rent amount input field to LocationEditForm with decimal display
- Update all database operations to persist rentAmount
- Add localization strings for both English and Croatian
- Fix missing notes field in insertOne/insertMany operations

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

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

messages/en.json

@@ -142,6 +142,8 @@
     "auto-rent-notification-info": "This option enables automatic sending of monthly rent bill to the tenant via email on the specified day of the month.",
     "auto-rent-notification-toggle-label": "send rent notification",
     "rent-due-day-label": "Day of month when rent is due",
+    "rent-amount-label": "Monthly rent amount",
+    "rent-amount-placeholder": "Enter rent amount",
     "tenant-email-legend": "TENANT EMAIL",
     "tenant-email-placeholder": "Enter tenant's email",
     "warning-missing-tenant-names": "Warning: Tenant first and last name are missing. The 2D barcode will not be displayed to the tenant when they open the shared link until both fields are filled in.",
@@ -161,6 +163,8 @@
       "tenant-last-name-required": "tenant last name is missing",
       "tenant-email-required": "tenant email is missing",
       "tenant-email-invalid": "email address is invalid",
+      "rent-amount-required": "rent amount is required when rent notification is enabled",
+      "rent-amount-positive": "rent amount must be a positive number",
       "validation-failed": "Validation failed. Please check the form and try again."
     }
   },