feat(transcription): add data-test-search-input hook for E2E selectors

For issue #380. Adds an explicit Playwright selector attribute on the
mention search input so E2E tests target a stable hook instead of a
fragile CSS class string.

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

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

@@ -149,6 +149,7 @@ function selectItem(item: Person) {
 			<input
 				id="mention-search"
 				type="search"
+				data-test-search-input
 				class="min-h-[44px] w-full bg-transparent font-sans text-sm text-ink placeholder:text-ink-3 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy focus-visible:ring-inset"
 				placeholder={m.person_mention_search_prompt()}
 				bind:value={searchQuery}