fix: IMAP-Konto bearbeiten + Löschen auch bei sync_running

- Store: UpdateCredentials() — Zugangsdaten + Passwort neu verschlüsseln,
  setzt status='idle', error_msg='', sync_running=false zurück
- Handler: PATCH /api/imap/{id} unterstützt nun Credential-Update
  (name/host/username vorhanden = Credential-Update, sonst sync_interval)
- Frontend: "Bearbeiten"-Button öffnet Edit-Dialog mit allen Feldern;
  Passwort-Feld leer = unverändertes Passwort
- Frontend: Löschen-Button nicht mehr durch sync_running blockiert
  (nur noch bei status=running gesperrt)

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

internal/imap/store.go

@@ -282,6 +282,33 @@ func (s *Store) UpdateDone(ctx context.Context, id int64, count int) error {
 	return nil
 }
 
+// UpdateCredentials updates the connection details and optionally the password
+// of an IMAP account. Pass an empty password to leave it unchanged.
+func (s *Store) UpdateCredentials(ctx context.Context, id int64, acc Account, password string) error {
+	if password != "" {
+		enc, err := encryptPassword(password, s.encKey)
+		if err != nil {
+			return fmt.Errorf("imap store: encrypt password: %w", err)
+		}
+		_, err = s.pool.Exec(ctx,
+			`UPDATE imap_accounts SET name=$1, host=$2, port=$3, tls=$4, username=$5, password_enc=$6,
+			 status='idle', error_msg='', sync_running=false WHERE id=$7`,
+			acc.Name, acc.Host, acc.Port, acc.TLS, acc.Username, enc, id)
+		if err != nil {
+			return fmt.Errorf("imap store: update credentials: %w", err)
+		}
+	} else {
+		_, err := s.pool.Exec(ctx,
+			`UPDATE imap_accounts SET name=$1, host=$2, port=$3, tls=$4, username=$5,
+			 status='idle', error_msg='', sync_running=false WHERE id=$6`,
+			acc.Name, acc.Host, acc.Port, acc.TLS, acc.Username, id)
+		if err != nil {
+			return fmt.Errorf("imap store: update credentials (no pw): %w", err)
+		}
+	}
+	return nil
+}
+
 // UpdateSyncInterval sets the automatic sync interval for an account.
 // intervalMin == 0 disables automatic sync.
 func (s *Store) UpdateSyncInterval(ctx context.Context, id int64, intervalMin int) error {