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).

internal/ui/handlers_audit.go

@@ -86,7 +86,7 @@ func (u *UIServer) handleAuditDetail(w http.ResponseWriter, r *http.Request) {
 	}
 
 	u.render(w, "audit_detail", AuditDetailData{
-		PageData: PageData{CSRFToken: csrfToken},
+		PageData: PageData{CSRFToken: csrfToken, ActorName: u.actorName(r)},
 		Event:    event,
 	})
 }
@@ -116,7 +116,7 @@ func (u *UIServer) buildAuditData(r *http.Request, page int, csrfToken string) (
 	}
 
 	return AuditData{
-		PageData:   PageData{CSRFToken: csrfToken},
+		PageData:   PageData{CSRFToken: csrfToken, ActorName: u.actorName(r)},
 		Events:     events,
 		EventTypes: auditEventTypes,
 		FilterType: filterType,