feat: @mentions in comments

Parse @username in comment content, notify mentioned users (dedup
against recipe-author/parent-author who are already notified via
comment/reply), and render @username as a link to their profile.

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

apps/web/lib/mentions.ts

@@ -0,0 +1,11 @@
+const MENTION_REGEX = /@([a-z0-9_-]{3,30})/gi;
+
+export function extractMentionedUsernames(content: string): string[] {
+  const matches = content.matchAll(MENTION_REGEX);
+  const usernames = new Set<string>();
+  for (const m of matches) {
+    const username = m[1];
+    if (username) usernames.add(username.toLowerCase());
+  }
+  return [...usernames];
+}