Add automatic rent notification feature to LocationEditForm

- Add rentDueNotification toggle to enable automatic rent notifications
- Add rentDueDay selector (1-28) for specifying when rent is due
- Extract tenant email to independent section shown when either autoBillFwd or rentDueNotification is enabled
- Update email validation to be mandatory when any automatic notification is active
- Update database schema to persist rentDueNotification and rentDueDay fields
- Add all database operations to handle new fields with proper defaults
- Add localization strings for English and Croatian

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

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

app/lib/db-types.ts

@@ -55,6 +55,10 @@ export interface BillingLocation {
     tenantEmail?: string | null;
     /** (optional) bill forwarding strategy */
     billFwdStrategy?: "when-payed" | "when-attached" | null;
+    /** (optional) whether to automatically send rent notification */
+    rentDueNotification?: boolean | null;
+    /** (optional) day of month when rent is due (1-31) */
+    rentDueDay?: number | null;
 };
 
 export enum BilledTo {