fix: prevent [object Object] rendering in 25 frontend components

Replace direct {error} JSX rendering with typeof check + .message fallback.
When error is an object (not a string), React showed [object Object].
Now renders error.message or fallback string.

frontend/src/components/calendar/IcsControls.tsx

@@ -138,7 +138,7 @@ export function IcsControls({ calendar, onImported }: IcsControlsProps) {
       )}
       {error && (
         <span className="text-xs text-danger-700" data-testid="ics-error">
-          {error}
+          {typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}
         </span>
       )}
     </div>

frontend/src/components/calendar/ResourceBooking.tsx

@@ -112,7 +112,7 @@ export function ResourceBooking({ entry, resources, onBooked }: ResourceBookingP
           className="text-xs text-danger-700 bg-danger-50 border border-danger-200 rounded-md p-2"
           data-testid="resource-error"
         >
-          {error}
+          {typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}
         </div>
       )}
     </div>

frontend/src/components/calendar/SharingSettings.tsx

@@ -141,7 +141,7 @@ export function SharingSettings({ calendar }: SharingSettingsProps) {
           className="text-xs text-danger-700 bg-danger-50 border border-danger-200 rounded-md p-2"
           data-testid="sharing-error"
         >
-          {error}
+          {typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}
         </div>
       )}
       {message && (

frontend/src/components/calendar/TaskDetailPanel.tsx

@@ -212,7 +212,7 @@ export function TaskDetailPanel({ entry, onClose, onChanged }: TaskDetailPanelPr
             className="text-sm text-danger-700 bg-danger-50 border border-danger-200 rounded-md p-2"
             data-testid="task-detail-error"
           >
-            {error}
+            {typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}
           </div>
         )}
       </div>