Add EmailTemplateAdmin page component (list + chrome + ThemeProvider)

[patoperpetua]

Parent: #138
Depends on: #139

Goal

Export EmailTemplateAdmin as the primary embeddable admin page from @singleton-sd/post-kit-editor: template list, ported EmailBuilder MUI canvas, PostKit metadata/variables/preview/validation/save/send chrome, and an internal MUI ThemeProvider so hosts need no MUI setup.

Scope

Public API

<EmailTemplateAdmin
  templates={catalog}
  onSave={saveToConsumerApi}
  onSendTest={postSendTestToBff} // optional
/>

• Absorb list/select behaviour from examples/admin-editor/App.tsx.
• Reuse existing save/send/validation/preview logic from EmailTemplateEditor.
• Mount ThemeProvider inside the component.
• Keep EmailTemplateEditor / EmailBuilderCanvas exported for advanced hosts.

Layout

Full-page layout: template list + PostKit sidebar chrome + EmailBuilder inspector/samples/canvas + sandboxed preview.

Tests / Storybook

• SSR smoke for admin shell with fixtures.
• Storybook story for EmailTemplateAdmin.

Constraints

• No credentials props; no disk/Git/network inside the package.
• Do not rewrite examples/admin-editor docs beyond what is needed to compile against the new export (docs/example thinning is the next issue).

Acceptance criteria

• EmailTemplateAdmin is exported from package index.
• Mounting with templates + onSave shows list, MUI canvas/inspector/samples, and PostKit chrome.
• Omit onSendTest hides Send-test; provide it shows Send-test.
• Host does not need to install or wrap MUI ThemeProvider.
• pnpm --filter @singleton-sd/post-kit-editor test and Storybook build succeed.

Agent implementation notes

Depends on the MUI canvas port. Branch: feat/<N>-email-template-admin.