Add "Paste from clipboard" button to page and markup sandboxes

[Copilot]

Both sandbox editors lacked a way to bulk-replace the editor content from the clipboard and immediately preview the result.

Changes

• Both sandbox templates (sandbox-markup.html, sandbox-page.html): Inject a "Paste from clipboard" button next to the Preview button. On click, reads the clipboard via navigator.clipboard.readText(), replaces the full CodeMirror 6 editor content, then triggers Preview. Button is only added to the DOM when the Clipboard API is available (secure context / HTTPS).

navigator.clipboard.readText().then(function(text) {
  const view = rlCodemirror.EditorView.findFromDOM(
    document.querySelector('#div_id_content .cm-editor')
  );
  view.dispatch(view.state.update({
    changes: { from: 0, to: view.state.doc.length, insert: text },
  }));
  previewBtn.click();
});

• sandbox-page.html (bugfix): The existing force-focus script was using the dead CodeMirror 5 API (div.CodeMirror / this.CodeMirror.focus()). Updated to use the CM6 API (EditorView.findFromDOM) consistent with the rest of the codebase.