fix(ui): use sort arrows ↑↓ instead of chevrons on sort button

Chevrons indicate collapsible elements, not sort direction. Match
the document search SortDropdown pattern using ↑/↓ text arrows.

Refs: #179

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

frontend/src/routes/briefwechsel/CorrespondenzPersonBar.svelte

@@ -152,35 +152,9 @@ function handleSuggestionSelect(id: string) {
 		class="flex items-center gap-2 border border-line bg-muted px-4 py-2.5 text-sm font-bold tracking-wide text-ink-2 uppercase transition hover:bg-muted hover:text-ink"
 	>
 		{#if sortDir === 'ASC'}
-			<svg
+			{m.conv_strip_sort_oldest()} ↑
-				xmlns="http://www.w3.org/2000/svg"
-				width="14"
-				height="14"
-				viewBox="0 0 24 24"
-				fill="none"
-				stroke="currentColor"
-				stroke-width="2.5"
-				stroke-linecap="round"
-				stroke-linejoin="round"
-				aria-hidden="true"
-				class="h-4 w-4"><polyline points="18 15 12 9 6 15" /></svg
-			>
-			{m.conv_strip_sort_oldest()}
 		{:else}
-			<svg
+			{m.conv_strip_sort_newest()} ↓
-				xmlns="http://www.w3.org/2000/svg"
-				width="14"
-				height="14"
-				viewBox="0 0 24 24"
-				fill="none"
-				stroke="currentColor"
-				stroke-width="2.5"
-				stroke-linecap="round"
-				stroke-linejoin="round"
-				aria-hidden="true"
-				class="h-4 w-4"><polyline points="6 9 12 15 18 9" /></svg
-			>
-			{m.conv_strip_sort_newest()}
 		{/if}
 	</button>