a11y(transcription): enlarge @mention magnifier and darken contrast

Bump h-4 w-4 to h-5 w-5 and text-ink-3 to text-ink-2 so the icon
carries enough visual weight to identify the input region without a
visible text label. Leonie FINDING-MENTION-001 on PR #629.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.svelte

@@ -157,7 +157,7 @@ function selectItem(item: Person) {
 				fill="none"
 				stroke="currentColor"
 				stroke-width="2"
-				class="h-4 w-4 shrink-0 text-ink-3"
+				class="h-5 w-5 shrink-0 text-ink-2"
 			>
 				<circle cx="11" cy="11" r="7" />
 				<path d="m20 20-3.5-3.5" stroke-linecap="round" />