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/components/AuthGuard.tsx

@@ -0,0 +1,46 @@
+import { useEffect, ReactNode } from "react";
+import { View, ActivityIndicator } from "react-native";
+import { Redirect } from "expo-router";
+import { useAuthStore } from "../stores";
+import { useThemeStore } from "../stores/ThemeStore";
+
+type AuthGuardProps = {
+  children: ReactNode;
+};
+
+/**
+ * Wraps content that requires authentication.
+ * - Loads stored user on mount
+ * - Shows loading indicator while checking auth state
+ * - Redirects to login if not authenticated
+ * - Renders children if authenticated
+ */
+export const AuthGuard = ({ children }: AuthGuardProps) => {
+  const { theme } = useThemeStore();
+  const { isAuthenticated, isLoading, loadStoredUser } = useAuthStore();
+
+  useEffect(() => {
+    loadStoredUser();
+  }, [loadStoredUser]);
+
+  if (isLoading) {
+    return (
+      <View
+        style={{
+          flex: 1,
+          justifyContent: "center",
+          alignItems: "center",
+          backgroundColor: theme.primeBg,
+        }}
+      >
+        <ActivityIndicator size="large" color={theme.chatBot} />
+      </View>
+    );
+  }
+
+  if (!isAuthenticated) {
+    return <Redirect href="/login" />;
+  }
+
+  return <>{children}</>;
+};