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/changelog.ts

@@ -1,5 +1,5 @@
 // Mirrors CHANGELOG.md at the repo root — update both together.
-export const APP_VERSION = "0.15.0";
+export const APP_VERSION = "0.16.0";
 
 export type ChangelogEntry = {
   version: string;
@@ -11,6 +11,13 @@ export type ChangelogEntry = {
 };
 
 export const CHANGELOG: ChangelogEntry[] = [
+  {
+    version: "0.16.0",
+    date: "2026-07-13 21:59",
+    added: [
+      "**@mention autocomplete in comments**: type \"@\" and a name to search and insert a mention, instead of typing the exact username blind.",
+    ],
+  },
   {
     version: "0.15.0",
     date: "2026-07-13 21:54",