UI: pgcreds create button; show logged-in user

* web/templates/pgcreds.html: New Credentials card is now always
  rendered; Add Credentials toggle button reveals the create form
  (hidden by default). Shows a message when all system accounts
  already have credentials. Previously the card was hidden when
  UncredentialedAccounts was empty.
* internal/ui/ui.go: added ActorName string field to PageData;
  added actorName(r) helper resolving username from JWT claims
  via DB lookup, returns empty string if unauthenticated.
* internal/ui/handlers_*.go: all full-page PageData constructors
  now pass ActorName: u.actorName(r).
* web/templates/base.html: nav bar renders actor username as a
  muted label before the Logout button when logged in.
* web/static/style.css: added .nav-actor rule (muted grey, 0.85rem).

web/templates/base.html

@@ -15,6 +15,8 @@
       <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>
+      {{if .ActorName}}<li><span class="nav-actor">{{.ActorName}}</span></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>
   </div>