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>

test/setup.ts

@@ -0,0 +1,10 @@
+// jsdom has no layout: give the few APIs the editor touches harmless defaults.
+if (!('scrollIntoView' in Element.prototype)) (Element.prototype as any).scrollIntoView = () => {}
+;(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true
+
+// Store updates arrive from fetch/timers outside act(); the harness settles them explicitly.
+const error = console.error
+console.error = (...a: unknown[]) => {
+  if (typeof a[0] === 'string' && a[0].includes('not wrapped in act')) return
+  error(...a)
+}