Replaced one-way checked={...} with bind:group={selected} driven by a
writable $derived. In Svelte 5, the $derived pattern guarantees the DOM
checked state is always in sync at FormData capture time, so groupIds
is never accidentally sent as [] when the admin edits their own profile.
Sending groupIds:[] causes adminUpdateUser to clear all groups, which
revokes the admin's own permissions on the next request.
Tests: UserServiceTest (+4 for adminUpdateUser group behaviour),
page.svelte.spec.ts (+1 FormData assertion at submit time).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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