fix(UI-Overhaul-Phase5): Kalender Visibility-Toggle fix

5.1: Toolbar already has PluginToolbar with navigation, view mode, actions — no changes needed
5.2: Calendar visibility toggle fix:
  - calendarStore.setCalendars now initializes visibleCalendarIds with all calendar IDs
  - CalendarTree.tsx visibility check simplified to visibleCalendarIds.has(cal.id)
  - No more empty-set-means-all-visible confusion

tsc clean

frontend/src/components/calendar/CalendarTree.tsx

@@ -128,9 +128,7 @@ function TypeSection({
               <CalendarRow
                 calendar={cal}
                 selected={activeCalendarId === cal.id}
-                visible={
-                  visibleCalendarIds.size === 0 || visibleCalendarIds.has(cal.id)
-                }
+                visible={visibleCalendarIds.has(cal.id)}
                 onSelect={() => onSelect(cal.id)}
                 onToggleVisibility={() => onToggleVisibility(cal.id)}
               />

frontend/src/store/calendarStore.ts

@@ -139,7 +139,13 @@ export const useCalendarStore = create<CalendarState>((set) => ({
     }),
 
   setActiveCalendarId: (id) => set({ activeCalendarId: id }),
-  setCalendars: (calendars) => set({ calendars }),
+  setCalendars: (calendars) => set((s) => {
+    // Initialize visibleCalendarIds with all calendar IDs if empty
+    const visibleCalendarIds = s.visibleCalendarIds.size === 0
+      ? new Set(calendars.map((c) => c.id))
+      : s.visibleCalendarIds;
+    return { calendars, visibleCalendarIds };
+  }),
   setSelectedEntry: (entry) => set({ selectedEntry: entry }),
   toggleCalendarVisibility: (id) =>
     set((s) => {