refactor: replace generateTenantCode boolean with tenantPaymentMethod enum

- Replace generateTenantCode boolean field with tenantPaymentMethod enum ("none" | "iban" | "revolut")
- Update LocationEditForm to use dropdown select instead of toggle for payment method selection
- Consolidate multiple useState hooks into single formValues state object
- Change from defaultValue to controlled components with value/onChange pattern
- Add hidden inputs to preserve tenant data when payment method is not selected
- Update validation logic to check tenantPaymentMethod === "iban"
- Update ViewLocationCard to use new tenantPaymentMethod field
- Add Croatian translations for new dropdown options

This provides better scalability for adding future payment methods and improves form state management.

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

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

app/lib/db-types.ts

@@ -51,8 +51,10 @@ export interface BillingLocation {
     bills: Bill[];
     /** (optional) notes */
     notes: string|null;
-    /** (optional) whether to generate 2D code for tenant */
-    generateTenantCode?: boolean | null;
+
+    /** (optional) method for showing payment instructions to tenant */
+    tenantPaymentMethod?: "none" | "iban" | "revolut" | null;
+
     /** (optional) tenant name */
     tenantName?: string | null;
     /** (optional) tenant street */