Add fallback preview size for DOM-only reference examples

[aashu2006]

Addresses #1204

Example previews on the reference site currently rely on detecting canvas dimensions via createCanvas().

For DOM-only examples (e.g. createCapture() used with noCanvas()), no canvas dimensions are detected, causing the preview container to collapse or clip the output.

This PR adds a fallback preview size (400×300) when both previewWidth and previewHeight remain undefined. This ensures DOM-based examples render a visible preview.

Changes

• Add fallback preview size (400×300) when no canvas dimensions are detected
• Keep existing behavior for sketches that use createCanvas()
• Place fallback after the DOM heuristic and before the largeSketch calculation

[aashu2006]

Hi @ksen0 @davepagurek

As discussed this PR adds a fallback preview size when no canvas dimensions are detected.
DOM-only examples like createCapture() with noCanvas() were collapsing because previewWidth and previewHeight stayed undefined.

The change introduces a 400×300 fallback so those examples render a visible preview, while sketches that use createCanvas() continue to use their detected size.

Let me know if this direction looks good or if you would prefer a different fallback approach.

[ksen0]

Thanks! Just 2 small things. Also, if possible, could you please revert unrelated changes in this and future PRs? It's harder to review when there's additional spacing/etc lines changed, so it would be really nice to exclude these. With things like regex especially having a more reliable gitblame is also good

[aashu2006]

Thanks for the review @ksen0 💜
The unrelated formatting changes happened due to the formatter running locally when I saved the file. I’ve now reverted those so the diff only contains the relevant logic changes. Let me know if everything looks good now or if anything else should be adjusted.

[ksen0]

I'll merge this one to fix the DOM-only examples, but I've changed the PR as "Addresses" because the other examples are still not showing up right (unrelated to size)

[aashu2006]

Yes, this PR only targets the DOM only cases where no canvas size is detected which was causing the preview container to collapse. The remaining examples already define a canvas size, so their rendering issue seems to be coming from a different part of the preview pipeline rather than the sizing logic. I am still figuring it out, will update further.