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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.15.0",
+  "version": "0.16.0",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",