Fix SEC-09: hide admin nav links from non-admin users

- Add IsAdmin bool to PageData (embedded in all page view structs)
- Remove redundant IsAdmin from DashboardData
- Add isAdmin() helper to derive admin status from request claims
- Set IsAdmin in all page-level handlers that populate PageData
- Wrap admin-only nav links in base.html with {{if .IsAdmin}}
- Add tests: non-admin dashboard/profile hide admin links,
  admin dashboard shows them

Security: navigation links to /accounts, /audit, /policies,
and /pgcreds are now only rendered for admin users. Server-side
authorization (requireAdminRole middleware) was already in place;
this change removes the information leak of showing links that
return 403 to non-admin users.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/templates/base.html

@@ -12,10 +12,10 @@
     <span class="nav-brand">MCIAS</span>
     <ul class="nav-links">
       <li><a href="/dashboard">Dashboard</a></li>
-      <li><a href="/accounts">Accounts</a></li>
+      {{if .IsAdmin}}<li><a href="/accounts">Accounts</a></li>
       <li><a href="/audit">Audit</a></li>
       <li><a href="/policies">Policies</a></li>
-      <li><a href="/pgcreds">PG Creds</a></li>
+      <li><a href="/pgcreds">PG Creds</a></li>{{end}}
       {{if .ActorName}}<li><a href="/profile">{{.ActorName}}</a></li>{{end}}
       <li><form method="POST" action="/logout" style="margin:0"><button class="btn btn-sm btn-secondary" type="submit">Logout</button></form></li>
     </ul>