feat(persons): show merge panel inline on edit page, remove Gefahrenzone accordion

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>

frontend/src/routes/persons/[id]/PersonMergePanel.svelte

@@ -15,7 +15,7 @@ let mergeTargetId = $state('');
 let showMergeConfirm = $state(false);
 </script>
 
-<div class="mb-10 overflow-hidden rounded-sm border border-line bg-surface shadow-sm">
+<div class="mb-10 overflow-hidden rounded-sm border border-red-200 bg-surface shadow-sm">
 	<div class="p-6 md:p-8">
 		<h2 class="mb-1 font-serif text-lg text-ink">{m.person_merge_heading()}</h2>
 		<p class="mb-5 font-sans text-sm text-ink-2">