feat(ui): two render states — hero vs results — with unified padding

Hero state (no senderId): centred CorrespondenzHero with discovery
headline, cross-link, large typeahead, recent persons. No person bar
or filter controls shown. Results state (senderId set): full-width
strips then content area with max-w-7xl responsive padding matching
other overview pages. Removes focus delegation hack.

Refs: #179

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/briefwechsel/CorrespondenzPersonBar.svelte

@@ -53,7 +53,10 @@ function handleSuggestionSelect(id: string) {
 }
 </script>
 
-<div class="flex items-end gap-[9px] border-b border-line bg-surface px-4 py-[9px] sm:px-[18px]">
+<div
+	data-testid="conv-person-bar"
+	class="flex items-end gap-[9px] border-b border-line bg-surface px-4 py-[9px] sm:px-[18px]"
+>
 	<!-- Person A -->
 	<div class="min-w-0 flex-1">
 		<PersonTypeahead