Add admin panel restricted to a single designated account

Adds a full admin view (users + projects, with view/delete) gated
server-side by ADMIN_EMAIL in server/auth.js (defaults to the site
owner's account, overridable via env var for other deployments). The
gate is enforced on every /api/admin/* route, not just hidden in the
UI — verified a non-admin session gets 403 even when it hits the
endpoints directly. Deleting a user leaves their projects in place
(not cascade-deleted) so admin cleanup can't accidentally destroy
someone's study data.

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

.gitignore

@@ -9,3 +9,4 @@ coverage/
 .vite.log
 .api.pid
 .api.log
+.DS_Store