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).

.junie/memory/language.json

@@ -0,0 +1 @@
+[]

.junie/memory/memory.version

@@ -0,0 +1 @@
+1.0