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/api/import_handlers.go

@@ -273,7 +273,9 @@ func (s *Server) handleSyncNow(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, acc)
 }
 
-// handleUpdateImapInterval updates the automatic sync interval for an IMAP account.
+// handleUpdateImapInterval updates the automatic sync interval or full credentials
+// of an IMAP account. When name/host/username are present the credentials are updated;
+// otherwise only sync_interval_min is changed.
 func (s *Server) handleUpdateImapInterval(w http.ResponseWriter, r *http.Request) {
 	if s.imapStore == nil {
 		writeError(w, http.StatusServiceUnavailable, "IMAP not configured")
@@ -299,13 +301,48 @@ func (s *Server) handleUpdateImapInterval(w http.ResponseWriter, r *http.Request
 	}
 
 	var req struct {
-		SyncIntervalMin int `json:"sync_interval_min"`
+		SyncIntervalMin int    `json:"sync_interval_min"`
+		Name            string `json:"name"`
+		Host            string `json:"host"`
+		Port            int    `json:"port"`
+		TLS             string `json:"tls"`
+		Username        string `json:"username"`
+		Password        string `json:"password"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, http.StatusBadRequest, "invalid request body")
 		return
 	}
 
+	// Credential update when name/host/username are provided.
+	if req.Name != "" || req.Host != "" || req.Username != "" {
+		if req.Name == "" || req.Host == "" || req.Username == "" {
+			writeError(w, http.StatusBadRequest, "name, host and username are required for credential update")
+			return
+		}
+		port := req.Port
+		if port == 0 {
+			port = acc.Port
+		}
+		tls := req.TLS
+		if tls == "" {
+			tls = acc.TLS
+		}
+		updated := imapstore.Account{Name: req.Name, Host: req.Host, Port: port, TLS: tls, Username: req.Username}
+		if err := s.imapStore.UpdateCredentials(r.Context(), id, updated, req.Password); err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to update credentials")
+			return
+		}
+		acc, err = s.imapStore.Get(r.Context(), id)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "failed to reload account")
+			return
+		}
+		writeJSON(w, http.StatusOK, acc)
+		return
+	}
+
+	// Sync interval update.
 	// 0 = disabled; otherwise must be between 5 and 1440 minutes.
 	if req.SyncIntervalMin != 0 && (req.SyncIntervalMin < 5 || req.SyncIntervalMin > 1440) {
 		writeError(w, http.StatusBadRequest, "sync_interval_min must be 0 (disabled) or between 5 and 1440")