Follow-up: isolate EmailBuilder editor Zustand store per surface

[patoperpetua]

Context

PR #142 ports the EmailBuilder.js MUI sample with a module-level Zustand singleton (editorStateStore). That matches the upstream sample and PostKit’s supported embed model: a single EmailTemplateAdmin / EmailBuilderMuiSurface per page.

CodeRabbit flagged that mounting two editable surfaces would share document/selection/drawers and could cross-fire onChange.

Goal

• Create the store per surface instance
• Provide via React context
• Bind hooks/actions to the provider
• Add a regression test: two canvases, edit one, other document + callback unchanged

Non-goals

• Redesigning the admin catalog UX
• Changing the public EmailTemplateAdmin API beyond what isolation requires

Parent: #138
Related: #142