feat: #3567225 Allow per-node override of Content Template via checkbox in node selector

Closes #3567225

Summary

Allows editing a Canvas layout on an individual node, overriding its Content Template.

• A per-node Canvas editor (/canvas/editor/node/{node}) edits the node's own layout.
• Field linking (dynamic prop sources) resolves against the node as the host entity. When editing a Content Template, the preview entity remains the host.
• The Page Data panel is hidden in the per-node editor, and when drupalSettings.canvas.hidePageDataPanel is TRUE.
• Prop-source suggestions are offered for node layouts, not only Content Templates.

Changes

• Back end: ApiLayoutController selects the host entity per context; JsonSchemaPropsComponentSourceBase suggests prop sources for nodes; ComponentTreeLoader is made extensible.
• Front end: ContextualPanel hides the Page Data panel; layoutModelSlice links props using the active editor frame context; Topbar/UndoRedo icon sizing; new hidePageDataPanel setting in DrupalSettings.

Testing

• Open /canvas/editor/node/{node} for a node that uses a Content Template.
• Confirm the Page Data panel is hidden.
• Add a component, link a prop to a node field, and save.
• Reload and confirm the linked value renders from the node.
• Edit the Content Template and confirm template editing still works.
• composer run lint and npm run lint pass.

AI usage disclosure

Per the Drupal AI contribution policy: AI (Claude Code, Opus 4.8) assisted with rebasing this branch onto the latest 1.x and resolving coding-standard, static-analysis, architecture-rule, and formatting violations (PHPCS, PHPStan, PHPat, ESLint, Prettier). All changes were reviewed and verified by the human contributor (Rajab Natshah), who is responsible for the logic, security, and licensing.