fix: don't copy theme layout.html to public output

[lucasnevespereira]

Summary

copyTheme was writing layout.html to public/themes/<name>/layout.html alongside the actual assets. The layout is a server-side template — it's never referenced by the browser and shouldn't ship.

Now only styles.css and script.js are copied. Same fix applied to both the embedded-theme path and the local-theme path (skips layout.html explicitly).

Test plan

• go vet ./... clean
• Build examples/minimal — output public/themes/minimal/ contains only script.js and styles.css
• No regression: pages still render correctly (layout still resolved server-side)