Add on-page editor for the go-graph-cms

Injectable React editor that is side-loaded onto the statically rendered
page and edits through the CMS HTTP API (POST /api/edit).

- Shadow-root UI over server HTML; patches morphed in, undo/redo from
  server inverses (reuses @k6n/cms-editor EditClient/morph/markdown editor)
- Sign-in via go-auth device grant as the CMS client (aud matches), token
  refresh + 401 retry; live preview swapped in on load and on tab focus
- Schema-driven property form (arrays, maps, variants, $ref, validation)
- Every string property editable where it is rendered (value-bound inline
  fields and attribute chips), markdown WYSIWYG for marked fields
- Block palette + drag & drop with ghost drop targets, insertion slots,
  sort snapping between siblings (Alt to nest)
- x-style annotations: on-page padding/margin/gap handles, Style panel,
  wrap-in-layout; x-graph label/group for titles and grouping
- Dev workflow: Vite proxy to the live site (token injected server-side)
  or an offline in-memory mock CMS; jsdom test suite

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/main.ts

@@ -0,0 +1,11 @@
+// Production entry: served as /editor/onpage-editor.js and loaded by the page's
+// own loader only when the preview cookie is set (docs/onpage-editor.md §2).
+// It re-checks, so loading it by mistake never shows editor UI to visitors.
+import { boot } from './boot'
+
+export const previewEnabled = () => /(?:^|;\s*)cms_preview=1/.test(document.cookie)
+
+if (previewEnabled()) boot()
+
+export { boot }
+export * from './registry'