Fix Cortext editor integration gaps

[priethor]

Goal

Close the shell-specific gaps that remain after mounting Gutenberg's editor inside Cortext.

Scope

• Unsaved-changes UX when users navigate away, close a detail surface, or leave the shell with dirty content.
• Post lock UI for concurrent editing, using core editor components where they fit.
• Canvas layout parity: derive the page content layout instead of hardcoding the constrained layout.
• Save, status, and error handling for failed saves and retries.
• Curate which core Gutenberg blocks make sense for a knowledge base. Hide or unregister the ones that don't (Latest Posts, RSS, anything pulling external content).

Out of scope

• Reimplementing Gutenberg paste behavior, undo/redo, block selection, slash insertion, or common blocks.
• Collection row detail editing, which belongs to the row detail issue.