Fix layout width calculations (#9)

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## 💸 TL;DR
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The width was always two pixels off because the border is two pixels.
This is a lot more noticable now that we have `context.dimensions`.
Added a calc to make it right.

![CleanShot 2024-04-10 at 15 34
42@2x](https://github.com/reddit/play/assets/153219522/2ca24611-fee7-4f69-8193-be830f0f6677)


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src/elements/play-preview.ts

@@ -48,6 +48,8 @@ export class PlayPreview extends LitElement {
       border-radius: 16px;
       min-height: 320px;
       box-shadow: var(--shadow-xs);
+      /* Prevents the border from throwing off the context.dimensions calculation */
+      box-sizing: content-box;
 
       /* When the background is visible, the preview is loading. */
       background-color: var(--color-interactive-background);