Add refresh controls and notification support

- Add refresh button to product list items with spinning animation
- Add editable refresh interval dropdown on product detail page
- Add user profile dropdown with settings link in navbar
- Create Settings page for Telegram and Discord configuration
- Add per-product notification options (price drop threshold, back in stock)
- Integrate notifications into scheduler for automatic alerts
- Add notification service supporting Telegram Bot API and Discord webhooks

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -5,6 +5,7 @@ import Login from './pages/Login';
 import Register from './pages/Register';
 import Dashboard from './pages/Dashboard';
 import ProductDetail from './pages/ProductDetail';
+import Settings from './pages/Settings';
 
 function ThemeInitializer({ children }: { children: React.ReactNode }) {
   useEffect(() => {
@@ -101,6 +102,14 @@ function AppRoutes() {
           </ProtectedRoute>
         }
       />
+      <Route
+        path="/settings"
+        element={
+          <ProtectedRoute>
+            <Settings />
+          </ProtectedRoute>
+        }
+      />
       <Route path="*" element={<Navigate to="/" replace />} />
     </Routes>
   );