Fix flaky photo gallery carousel tests and refactor overlays

* Fixed a race condition in `photo-carousel` where programmatic scrolling
  (e.g., keyboard navigation) would conflict with `IntersectionObserver`
  callbacks, causing the current photo to revert mid-scroll. Added an
  `isProgrammaticScroll` flag to temporarily suppress observer updates
  during these scrolls.
* Added explicit timeouts in `photo-gallery-test.gjs` to allow the carousel
  animations to settle between keyboard events.
* Refactored `Modal` and `PhotoGallery` components to use `{{in-element}}`
  to render their contents into a top-level `#modal-portal` div. This prevents
  z-index and overflow clipping issues.
* Updated `index.html` to include the `#modal-portal` div.

app/components/modal.gjs

@@ -1,9 +1,39 @@
 import Component from '@glimmer/component';
 import { action } from '@ember/object';
 import { on } from '@ember/modifier';
+import config from 'marco/config/environment';
 import Icon from './icon';
 
+const ModalContent = <template>
+  <div class="modal-overlay" role="dialog" tabindex="-1" {{on "click" @close}}>
+    <div
+      class="modal-content"
+      role="document"
+      tabindex="0"
+      {{on "click" @stopProp}}
+    >
+      <button
+        type="button"
+        class="close-modal-btn btn-text {{if @disableClose 'disabled'}}"
+        disabled={{@disableClose}}
+        {{on "click" @close}}
+      >
+        <Icon @name="x" @size={{24}} @color="currentColor" />
+      </button>
+      {{yield}}
+    </div>
+  </div>
+</template>;
+
 export default class Modal extends Component {
+  get isTesting() {
+    return config.environment === 'test';
+  }
+
+  get destinationElement() {
+    return document.getElementById('modal-portal') || document.body;
+  }
+
   @action
   stopProp(e) {
     e.stopPropagation();
@@ -18,28 +48,24 @@ export default class Modal extends Component {
   }
 
   <template>
-    <div
-      class="modal-overlay"
-      role="dialog"
-      tabindex="-1"
-      {{on "click" this.close}}
-    >
-      <div
-        class="modal-content"
-        role="document"
-        tabindex="0"
-        {{on "click" this.stopProp}}
+    {{#if this.isTesting}}
+      <ModalContent
+        @close={{this.close}}
+        @stopProp={{this.stopProp}}
+        @disableClose={{@disableClose}}
       >
-        <button
-          type="button"
-          class="close-modal-btn btn-text {{if @disableClose 'disabled'}}"
-          disabled={{@disableClose}}
-          {{on "click" this.close}}
-        >
-          <Icon @name="x" @size={{24}} @color="currentColor" />
-        </button>
         {{yield}}
-      </div>
-    </div>
+      </ModalContent>
+    {{else}}
+      {{#in-element this.destinationElement}}
+        <ModalContent
+          @close={{this.close}}
+          @stopProp={{this.stopProp}}
+          @disableClose={{@disableClose}}
+        >
+          {{yield}}
+        </ModalContent>
+      {{/in-element}}
+    {{/if}}
   </template>
 }