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)}
               />