fix: improve modal behavior on web and Android scrolling

- Restructure ModalBase to use absolute-positioned backdrop behind card
  content, fixing modal stacking issues on web (React Native Web portals)
- Hide EventOverlay when DeleteEventModal is open to prevent z-index
  conflicts on web
- Add nestedScrollEnabled to CardBase ScrollView for Android
- Use TouchableOpacity with delayPressIn in EventCard for scroll-friendly
  touch handling
- Keep eventToDelete state stable during modal fade-out to prevent
  content flash between recurring/single variants
- Fix German umlauts in DeleteEventModal

apps/client/src/app/(tabs)/calendar.tsx

@@ -188,14 +188,14 @@ const Calendar = () => {
       }
     } catch (error) {
       console.error("Failed to delete event:", error);
-    } finally {
-      setEventToDelete(null);
     }
+    // Note: Don't clear eventToDelete here - it will be overwritten when opening a new modal.
+    // Clearing it during fade-out animation causes the modal content to flash from recurring to single.
   };
 
   const handleDeleteCancel = () => {
     setDeleteModalVisible(false);
-    setEventToDelete(null);
+    // Note: Don't clear eventToDelete - keeps modal content stable during fade-out animation
   };
 
   // Get events for selected date
@@ -222,7 +222,7 @@ const Calendar = () => {
         onDayPress={handleDayPress}
       />
       <EventOverlay
-        visible={selectedDate !== null}
+        visible={selectedDate !== null && !deleteModalVisible}
         date={selectedDate}
         events={selectedDateEvents}
         onClose={handleCloseOverlay}