refactor: add AuthGuard component and fix API client empty response handling

- Add reusable AuthGuard component for protected routes
- Move auth logic from index.tsx to (tabs)/_layout.tsx via AuthGuard
- Fix ApiClient to handle empty responses (204 No Content)
- Use useFocusEffect in chat.tsx to load messages after auth is ready
- Extract getDateKey() helper function in calendar.tsx

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

@@ -33,6 +33,14 @@ type MonthItem = {
   label: string; // e.g. "January 2024"
 };
 
+/**
+ * Formats a Date object to a string key in YYYY-MM-DD format.
+ * Used for grouping and looking up events by date.
+ */
+const getDateKey = (date: Date): string => {
+  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, "0")}-${String(date.getDate()).padStart(2, "0")}`;
+};
+
 const generateMonths = (
   centerYear: number,
   centerMonth: number,
@@ -108,7 +116,7 @@ const Calendar = () => {
     const map = new Map<string, ExpandedEvent[]>();
     events.forEach((e) => {
       const date = new Date(e.occurrenceStart);
-      const key = `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, "0")}-${String(date.getDate()).padStart(2, "0")}`;
+      const key = getDateKey(date);
       if (!map.has(key)) map.set(key, []);
       map.get(key)!.push(e);
     });
@@ -157,7 +165,7 @@ const Calendar = () => {
             deleteEvent(event.id);
             // Close overlay if no more events for this date
             if (selectedDate) {
-              const dateKey = `${selectedDate.getFullYear()}-${String(selectedDate.getMonth() + 1).padStart(2, "0")}-${String(selectedDate.getDate()).padStart(2, "0")}`;
+              const dateKey = getDateKey(selectedDate);
               const remainingEvents = eventsByDate.get(dateKey) || [];
               if (remainingEvents.length <= 1) {
                 setSelectedDate(null);
@@ -175,7 +183,7 @@ const Calendar = () => {
   // Get events for selected date
   const selectedDateEvents = useMemo(() => {
     if (!selectedDate) return [];
-    const key = `${selectedDate.getFullYear()}-${String(selectedDate.getMonth() + 1).padStart(2, "0")}-${String(selectedDate.getDate()).padStart(2, "0")}`;
+    const key = getDateKey(selectedDate);
     return eventsByDate.get(key) || [];
   }, [selectedDate, eventsByDate]);
 
@@ -597,10 +605,6 @@ const CalendarGrid = (props: CalendarGridProps) => {
     return date;
   };
 
-  const getDateKey = (date: Date): string => {
-    return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, "0")}-${String(date.getDate()).padStart(2, "0")}`;
-  };
-
   return (
     <View
       className="h-full flex-1 flex-col flex-wrap gap-2 p-2"