Add billedToTenant property to Bill interface with UI support

Added new boolean property to track whether a bill should be paid by the tenant.

Changes:
- Added billedToTenant property to Bill interface in db-types.ts
- Added checkbox UI control in BillEditForm for billedToTenant
- Added state management and change handler for the checkbox
- Added i18n translations (EN: "Billed to tenant", HR: "Plaća podstanar")
- Set default value to true for new bills

Note: Server action implementation pending - property not yet persisted to database.

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

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

messages/en.json

@@ -103,7 +103,8 @@
       "form-error-message": "Form validation error. Please check the form and try again."
     },
     "attachment": "Attachment",
-    "back-button": "Back"
+    "back-button": "Back",
+    "billed-to-tenant": "Billed to tenant"
   },
   "location-delete-form": {
     "text": "Please confirm deletion of realestate \"<strong>{name}</strong>\".",