Add PrincipalSelectorInput component to form2

[edloidas]

Posponed until app-users migration.

An async autocomplete combobox that searches users, groups, and roles from the server. The principalTypes config restricts which types appear in results; skipPrincipals excludes specific entries by key. Search is debounced (300ms) and shows a loading mask while fetching. If a previously saved principal has been deleted, it remains visible in the selection with a "This user is deleted" indicator rather than silently disappearing. Supports multi-select up to occurrences.max, and in the legacy selected items can be reordered.

Implementation

1. Create components/principal-selector-input/ with PrincipalSelectorInput.tsx, index.ts, PrincipalSelectorInput.test.ts, and PrincipalSelectorInput.stories.tsx
2. Re-export from components/index.ts
3. Register in initBuiltInComponents.ts via ComponentRegistry.register

_{Drafted with AI assistance}