fix(transcription): cap @mention search input at maxlength=100

Soft-cap on the client side mitigates CWE-400 query amplification
(server-side cap remains a separate backend PR).

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

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

@@ -166,6 +166,7 @@ function selectItem(item: Person) {
 				id="mention-search"
 				type="search"
 				data-test-search-input
+				maxlength="100"
 				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}