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/routes/briefwechsel/CorrespondenzPersonBar.svelte

@@ -102,6 +102,7 @@ function handleSuggestionSelect(id: string) {
 			stroke-linecap="round"
 			stroke-linejoin="round"
 			aria-hidden="true"
+			class="rotate-90"
 		>
 			<path d="M7 16V4m0 0L3 8m4-4l4 4" />
 			<path d="M17 8v12m0 0l4-4m-4 4l-4-4" />