fix(playground): handle trailing newlines correctly

[ohnishant]

Summary

• insert one source newline for each plain Enter keypress
• allow the editor to return to an empty source after deleting all content
• preserve autocomplete Enter behavior and Ctrl+Enter run behavior

Issues

Closes #35
Closes #36

Verification

• go test ./...
• go vet ./...
• gofmt -l .
• Chromium browser checks at desktop and iPhone 12 Pro viewports

The two fixes are separate commits so each issue can be reviewed independently.

[ohnishant]

Two things I want to flag before this lands.

First, the Enter handler does not check e.isComposing. When someone uses an IME, pressing Enter confirms the composition, and our preventDefault() plus newline insert would break that. I think we should add an early return for e.isComposing.

Second, the empty-editor sentinel in editorSource is tightly scoped. It only catches a single <br> or a single <div><br></div>. That is enough for the browser behavior I saw in testing, but if a browser leaves multiple empty block elements, we would still read a stray newline. Since we rebuild the DOM with buildEditorFragment on every edit, this is probably fine, but it is a narrow guard.

I will push a small follow-up for the IME case.

[ohnishant]

Superseded by #38. I had to rename the branch to follow the repo convention, so GitHub required a new PR.