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/db/accounts.go

@@ -450,16 +450,17 @@ func (db *DB) WritePGCredentials(accountID int64, host string, port int, dbName,
 func (db *DB) ReadPGCredentials(accountID int64) (*model.PGCredential, error) {
 	var cred model.PGCredential
 	var createdAtStr, updatedAtStr string
+	var ownerID sql.NullInt64
 
 	err := db.sql.QueryRow(`
 		SELECT id, account_id, pg_host, pg_port, pg_database, pg_username,
-		       pg_password_enc, pg_password_nonce, created_at, updated_at
+		       pg_password_enc, pg_password_nonce, created_at, updated_at, owner_id
 		FROM pg_credentials WHERE account_id = ?
 	`, accountID).Scan(
 		&cred.ID, &cred.AccountID, &cred.PGHost, &cred.PGPort,
 		&cred.PGDatabase, &cred.PGUsername,
 		&cred.PGPasswordEnc, &cred.PGPasswordNonce,
-		&createdAtStr, &updatedAtStr,
+		&createdAtStr, &updatedAtStr, &ownerID,
 	)
 	if errors.Is(err, sql.ErrNoRows) {
 		return nil, ErrNotFound
@@ -476,6 +477,10 @@ func (db *DB) ReadPGCredentials(accountID int64) (*model.PGCredential, error) {
 	if err != nil {
 		return nil, err
 	}
+	if ownerID.Valid {
+		v := ownerID.Int64
+		cred.OwnerID = &v
+	}
 	return &cred, nil
 }