Minor fixes post preview/iframe

List of fixes

• Scaffold options vanished after an undo or a change on the Canvas: the pane header now survives a reload of its pane.
• A disabled control in the Canvas, Bootstrap's disabled link too, could not be selected, right-clicked or outlined on hover: it is found from the pointer position.
• #3629497-11: Render the Canvas in an iframe on the front theme Placeholders were unreadable when the builder and the front theme differ in light or dark mode: they take the builder's text color.
• Shortcuts stopped after a click in the preview: its iframe forwards the keys, as the Canvas does.
• A panel rebuilt by the server lost the selection outline: it is marked again after the swap.
• The Instances panel hid Page layout, and its Add action, until one existed: shown to users who can add one.
• The Preview toggle tooltip covered the Canvas: removed.