Closes#342. The PersonDangerZone collapsible wrapper is removed; PersonMergePanel
is now rendered directly in the edit page with its own red border (border-red-200),
preserving the {#key person.id} state-reset behaviour and the two-step merge flow.
Fix PersonTypeahead mock to use Svelte 5 functional stub (not Svelte 3/4 $$ internals).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>