fix(playground): handle trailing newlines correctly

[ohnishant]

Fixes two related editor behaviors in the playground.

• One Enter keypress now inserts exactly one source newline, instead of the browser's default block markup creating two.
• Deleting all content leaves the editor truly empty, rather than a stranded <br> keeping the source as \n.

The Enter handler still leaves autocomplete and Ctrl+Enter run alone. I also added an e.isComposing guard so IME users can confirm composition with Enter.

Closes #35
Closes #36
Supersedes #37 (branch renamed to follow the fix/... convention).

Verification:

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