Style the editor surface: constrain width, add framing, respect dark mode

The bundled isolated-block-editor CSS hard-codes a full-width white
surface that ignores prefers-color-scheme, and the .skypress-editor
wrapper had no styles of its own. On large viewports the editor spanned
the full width, lacked padding, and rendered as a white block on the
dark page.

Constrain and frame it as a centered writing panel, and drive its
colours from the existing design tokens (--paper-raised, --line-strong,
--ink, --shadow) so it follows light/dark like the rest of the app. The
visual editor and Gutenberg's inline-styled device-preview wrapper are
made transparent so the paper surface shows through.