feat: refactor forms to use reactive forms with signals for improved state management

frontend/src/app/components/lease-form/lease-form.spec.ts

@@ -9,17 +9,19 @@ describe('LeaseForm', () => {
     fixture.componentRef.setInput('apartments', []);
     fixture.componentRef.setInput('tenants', []);
     const component = fixture.componentInstance;
-    component.apartmentId.set('apartment-1');
-    component.tenantIds.set(['tenant-1']);
-    component.startDate.set(new Date(2026, 8, 1));
-    component.endDate.set(new Date(2027, 7, 31));
-    component.monthlyRent.set(850);
-    component.monthlyCharges.set(50);
-    component.securityDeposit.set(850);
+    component.leaseModel.set({
+      apartmentId: 'apartment-1',
+      tenantIds: ['tenant-1'],
+      startDate: new Date(2026, 8, 1),
+      endDate: new Date(2027, 7, 31),
+      monthlyRent: 850,
+      monthlyCharges: 50,
+      securityDeposit: 850,
+    });
     let startDate = '';
     component.submitted.subscribe((lease) => (startDate = lease.startDate));
 
-    component.save({ invalid: false } as never);
+    component.save();
 
     expect(startDate).toBe('2026-09-01');
   });