fix(ui): De Gruyter long arrows on both sort buttons, rotate swap icon 90°

Replace ↑↓ text with Long-Arrow-Up/Down-MD.svg on the document search
SortDropdown and the Briefwechsel sort button. Rotate the swap button
SVG 90° so arrows point left/right matching the horizontal person
field layout.

Refs: #179

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

frontend/src/lib/components/SortDropdown.svelte

@@ -47,6 +47,20 @@ function toggleDir() {
 		class="-ml-px flex items-center justify-center border border-line bg-muted px-3 py-2.5 text-sm font-bold text-ink-2 transition hover:text-ink focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 		aria-label={dir === 'asc' ? m.sort_dir_asc() : m.sort_dir_desc()}
 	>
-		{dir === 'asc' ? '↑' : '↓'}
+		{#if dir === 'asc'}
+			<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}
+			<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>
 </div>