Refactor nested layout support

Now I have the full picture. Here's the issue text:

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Problem/Motivation

The lb_plus module is refactoring how nested layouts work — replacing the path-based nestedStoragePath system with a UUID-based NestedAwareSectionStorage decorator. The navigation_plus module had several dependencies on the old system:

• The edit-mode-plugin.js getSectionStorageInfo() method built a nestedStoragePath string by walking up the DOM tree through parent layout blocks — complex logic that is no longer needed.
• LoadEditablePage and EntityViewAlter depended on edit_plus's EntityConverter param converter for tempstore entity swapping, creating an implicit cross-module dependency.
• NavigationPlusUi only checked for OverridesSectionStorageInterface when resolving the current entity from route parameters, missing the NestedAwareSectionStorage decorator.

Proposed resolution

• Remove the nestedStoragePath building logic and elementIsNested() helper from edit-mode-plugin.js — nested routing is now handled transparently by lb_plus's NestedRouteEnhancer.
• Replace the dependency on edit_plus.param_converter.entity with tempstore_plus.param_converter.entity in LoadEditablePage and EntityViewAlter, removing the implicit edit_plus coupling.
• Add a NavigationPlusTempstoreActivationChecker that decorates tempstore_plus.activation_checker to gate tempstore swapping on edit mode being active.
• Update NavigationPlusUi to also recognize NestedAwareSectionStorage when resolving the current entity from route parameters.
• Update section-dropzone-wrappers.js to pass layoutBlockUuid for empty nested section handling.
• Add tempstore_plus as a module dependency.

API changes

• getSectionStorageInfo() in edit-mode-plugin.js no longer returns a nestedStoragePath property.
• navigation_plus.activation_checker service added (decorates tempstore_plus.activation_checker).