a11y(transcription): give @mention search input its own sr-only label

The sr-only label for the search input was reusing the listbox
"Link person" label — but the input filters a candidate list, it does
not link anything. Screen readers heard a verb mismatch between the
listbox announce and the search-input focus event. New
person_mention_search_label key in de/en/es. The listbox aria-label
stays person_mention_btn_label since that labels the listbox itself.
Leonie FINDING-MENTION-004 on PR #629.

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

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

@@ -164,7 +164,7 @@ function selectItem(item: Person) {
 	style:left={position.left}
 >
 	<div class="border-b border-line px-3 py-2">
-		<label class="sr-only" for="mention-search">{m.person_mention_btn_label()}</label>
+		<label class="sr-only" for="mention-search">{m.person_mention_search_label()}</label>
 		<div class="flex items-center gap-2">
 			<svg
 				aria-hidden="true"