feat(EntityGrid): implement infinite scroll for entity lists

Add infinite scroll functionality to EntityGrid component using VueUse's
useInfiniteScroll composable to handle large volumes of entities efficiently.

Changes:
- Integrate @vueuse/core useInfiniteScroll composable
- Add infinite scroll state management (displayedCount, reset function)
- Configure initial batch size (20 items) and increment size (20 items)
- Update displayedEntities, alphabeticalContacts to support progressive loading
- Add canLoadMore() logic for people, projects, and search modes
- Reset scroll state when search term or entities prop changes
- Remove maxItems prop (replaced by infinite scroll)
- Simplify displayEntitiesFunction signature (removed maxItems parameter)
- Update EntitySelectionStep and test files to remove max-items prop

Technical details:
- Uses template ref (scrollContainer) to access scrollable container
- Recent contacts (3) count toward initial batch for people grid
- Special entities (You, Unnamed) always displayed, don't count toward limits
- Infinite scroll works for both entity types and search results
- Constants are configurable at top of component (INITIAL_BATCH_SIZE, INCREMENT_SIZE)

This improves performance and UX when displaying large lists of contacts or
projects by loading content progressively as users scroll.

src/components/EntitySelectionStep.vue

@@ -15,7 +15,6 @@ properties * * @author Matthew Raymer */
     <EntityGrid
       :entity-type="shouldShowProjects ? 'projects' : 'people'"
       :entities="shouldShowProjects ? projects : allContacts"
-      :max-items="10"
       :active-did="activeDid"
       :all-my-dids="allMyDids"
       :all-contacts="allContacts"