Fix voice ball aspect ratio in fullscreen mode (#555)

Fixes #542

Add CSS rule to maintain the aspect ratio of the voice ball when entering fullscreen mode.

* **apps/style/edge.css**
  - Add CSS rule `#voiceBall { aspect-ratio: 1; }` to ensure the voice ball maintains its shape when entering fullscreen mode.

* **.devcontainer/devcontainer.json**
  - Add a new file with build task configuration.

---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/tjy-gitnub/win12/issues/542?shareId=XXXX-XXXX-XXXX-XXXX).

.devcontainer/devcontainer.json

@@ -0,0 +1,5 @@
+{
+  "tasks": {
+    "build": "# No build commands are necessary for this project."
+  }
+}

apps/style/edge.css

@@ -115,4 +115,8 @@
 .reloading>svg>circle:last-child {
     stroke: #2983cc;
     stroke-width: 2px;
-}
+}
+
+#voiceBall {
+    aspect-ratio: 1;
+}