refactor(editor): split non-site workspace layout state

[DavidBabinec]

What changed

• Added an admin-level workspace layout store for Content, Data, and Media sidebar/right-panel chrome.
• Moved per-workspace layout storage out of the Site editor folder into src/admin/state.
• Split Site editor layout persistence into a Site-only mapper/hook and removed Data/sidebar chrome state from uiSlice.
• Rewired Content, Data, Media, and AdminWorkspaceCanvasLayout away from @site/store/store.
• Added an architecture gate preventing non-site workspaces from importing the Site editor store.
• Updated docs and affected tests for the new ownership boundary.

Why

Content/Data/Media were depending on the full Site editor store for generic shell state. That made the store boundary blurry and kept non-site workspace chrome coupled to Site-only panel concepts. This refactor gives shared workspace chrome its own admin-level owner while keeping Site editor layout persistence explicitly Site-scoped.

Impact

The immediate benefit is a cleaner dependency boundary: non-site workspace layout behavior can change without touching the Site editor store, and future store refactors have an executable import gate. Content/Data/Media still preserve their sidebar widths, right-panel state, and Data sidebar collapse state through the same instatic-editor-layout-v2 key.

Verification

• bun run build
• bun run lint
• bun test
• bunx react-doctor@latest --verbose --scope changed --base main