Refactor nested layout support

Problem/Motivation

The lb_plus module is refactoring how nested layouts work — replacing the path-based nestedStoragePath system with a UUID-based NestedAwareSectionStorage decorator. This eliminates the need to thread a nested storage path through routes, JS, and forms.

Proposed resolution

• Move edit_plus_lb functionality (the Change tool's UpdateBlockForm, routing, and the block properties form alter) into edit_plus core, registered conditionally when lb_plus is available.
• Replace the EntityConverter param converter decorator with a lighter EditPlusTempstoreActivationChecker that integrates with tempstore_plus's activation checker API.
• Update BlockPropertiesEvent and LayoutBuilderBlockFormAlter to use lb_plus's new NestedSectionStorageInterface instead of the removed SectionStorageHandler.
• Remove nestedStoragePath URL handling from JS — nested routing is now handled transparently by lb_plus.
• Deprecate edit_plus_lb with an update hook to auto-uninstall.

API changes

• BlockPropertiesEvent constructor signature changed — accepts a single SectionStorageInterface instead of two OverridesSectionStorage parameters. getSectionStorage() replaces getMainSectionStorage() / getCurrentSectionStorage().
• edit_plus.param_converter.entity service replaced by edit_plus.activation_checker.