Content: let viewers pin shared sidebar pages - #2412

Incremental Code Review Summary

The latest PR head keeps the same eight-file sidebar change: shared action availability is centralized, viewer rows expose personal Pin/Unpin independently of shared Page authority, and both database and document-tree rows retain a stable overflow-before-add-child layout. I compared the refreshed diff and surrounding permission/action code against the prior clean review. The three parallel incremental reviews found no new confirmed regressions. One reviewer raised the existing canEdit !== false default semantics for absent optional fields; this is unchanged from the prior reviewed implementation and matches the established Content behavior for legacy/local records, so it is not a new incremental finding and is not resubmitted.

The latest focused tests and server-side checks continue to cover viewer-only menus, private favorite authorization, disabled-plus behavior, role regressions, ordering, and callback safety. No schema, shared authorization, or mutation contract changes were introduced.

🧪 Browser testing: Attempted after this review, but visual verification is infra-unavailable — /content returned 200 and the dev server was healthy, while all browser executors again lacked Chrome automation tools. Role fixtures and server-side viewer pin/unpin checks were prepared and verified for a later rerun.