fix(ui): use De Gruyter long arrow icons for sort direction

Replace tiny ↑↓ text with Long-Arrow-Up/Down-MD.svg icons from the
brand icon set for better visibility and consistency.

Refs: #179

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

frontend/src/routes/briefwechsel/CorrespondenzPersonBar.svelte

@@ -152,9 +152,21 @@ 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'}
-			{m.conv_strip_sort_oldest()} ↑
+			{m.conv_strip_sort_oldest()}
+			<img
+				src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Long-Arrow/Long-Arrow-Up-MD.svg"
+				alt=""
+				aria-hidden="true"
+				class="h-5 w-5 opacity-60"
+			/>
 		{:else}
-			{m.conv_strip_sort_newest()} ↓
+			{m.conv_strip_sort_newest()}
+			<img
+				src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Long-Arrow/Long-Arrow-Down-MD.svg"
+				alt=""
+				aria-hidden="true"
+				class="h-5 w-5 opacity-60"
+			/>
 		{/if}
 	</button>