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>

app/lib/db-types.ts

@@ -61,6 +61,8 @@ export interface BillingLocation {
     rentDueNotification?: boolean | null;
     /** (optional) day of month when rent is due (1-31) */
     rentDueDay?: number | null;
+    /** (optional) monthly rent amount in cents */
+    rentAmount?: number | null;
 };
 
 export enum BilledTo {