Add a Site Preview mode toggle to the canvas

## Goal

When you're prepping a page or row for publication, you want to see how it'll look on the public site, not how it looks in Cortext's editing theme. Add a toggle that swaps the canvas between the Cortext theme (Knowledge mode) and the active WP theme (Site Preview mode).

## Scope

* A toggle in the canvas header with Knowledge and Site Preview options.
* Knowledge mode: canvas renders under Cortext tokens (default).
* Site Preview mode: canvas renders under the active WP theme and theme.json.
* The chosen mode persists per user.

## Out of scope

* Actually publishing the page.
* Sharing or permissions.
* Public-side rendering itself; this only swaps the editor's view.

## Acceptance criteria

* The canvas shows a mode toggle with Knowledge and Site Preview.
* Knowledge mode shows the canvas under the Cortext theme.
* Site Preview mode shows the canvas under the active WP theme.
* The choice survives reload.

Start at the canvas header and its rendering path, then trace how Knowledge mode applies Cortext tokens and how the active WordPress theme and theme.json can be selected. Identify the per-user preference entry point and verify the acceptance criteria: both modes are available, each renders with the requested theme, and the choice survives reload.

Written by the indexing model from the issue text.