feat: @mention autocomplete in comment composer

Typing "@" + 2+ characters now opens a dropdown of matching users
(reusing the existing people-search endpoint), with arrow-key/Enter/
Tab selection and click-to-select. Consolidated the previously
duplicated MENTION_REGEX (client display vs. server extraction) into
a single export from lib/mentions.ts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/mentions.ts

@@ -1,4 +1,4 @@
-const MENTION_REGEX = /@([a-z0-9_-]{3,30})/gi;
+export const MENTION_REGEX = /@([a-z0-9_-]{3,30})/gi;
 
 export function extractMentionedUsernames(content: string): string[] {
   const matches = content.matchAll(MENTION_REGEX);