Add middle class styling and adjust button margin

Introduced a new .middle CSS class for vertical centering in theater-layout-designer component. Updated ticket-validation button margin from mt-3 to mt-2 for improved spacing.

src/app/theater-layout-designer/theater-layout-designer.component.css

@@ -0,0 +1,7 @@
+.middle {
+  position: relative;
+  top: 40%;
+  -webkit-transform: translateY(-50%);
+  -ms-transform: translateY(-50%);
+  transform: translateY(-50%);
+}

src/app/ticket-validation/ticket-validation.component.html

@@ -40,7 +40,7 @@
 
         <button
           mat-button
-          class="w-full mt-3"
+          class="w-full mt-2"
           matButton="filled"
           color="accent"
           [disabled]="(loadingService.loading$ | async) || (formControl.invalid && !formControl.hasError('serverError'))"