feat: Update Event Preview Modal UI with custom MUI theme (#240)

- Add custom MUI theme
- Update ResponsiveDialog with actionsBorderTop prop for modal footer border
- Redesign EventDisplayPreview modal with improved typography and spacing
- Replace ButtonGroup with individual RSVP buttons (Accept/Maybe/Decline)
- Add pill-shaped buttons for RSVP actions
- Ensure backward compatibility with existing modals

src/App.tsx

@@ -8,18 +8,22 @@ import { Loading } from "./components/Loading/Loading";
 import HandleLogin from "./features/User/HandleLogin";
 import CalendarLayout from "./components/Calendar/CalendarLayout";
 import { Error } from "./components/Error/Error";
+import { CustomThemeProvider } from "./theme/ThemeProvider";
+
 function App() {
   return (
-    <Suspense fallback={<Loading />}>
-      <Router history={history}>
-        <Routes>
-          <Route path="/" element={<HandleLogin />} />
-          <Route path="/calendar" element={<CalendarLayout />} />
-          <Route path="/callback" element={<CallbackResume />} />
-          <Route path="/error" element={<Error />} />
-        </Routes>
-      </Router>
-    </Suspense>
+    <CustomThemeProvider>
+      <Suspense fallback={<Loading />}>
+        <Router history={history}>
+          <Routes>
+            <Route path="/" element={<HandleLogin />} />
+            <Route path="/calendar" element={<CalendarLayout />} />
+            <Route path="/callback" element={<CallbackResume />} />
+            <Route path="/error" element={<Error />} />
+          </Routes>
+        </Router>
+      </Suspense>
+    </CustomThemeProvider>
   );
 }