feat: add loading page with real-time initialization progress

- Create loading.html template with WebSocket-based progress updates
- Update initialization_service to emit progress events via ProgressService
- Modify setup endpoint to run initialization in background and redirect to loading page
- Add /loading route in page_controller
- Show real-time progress for series sync, NFO scan, and media scan steps
- Display completion message with button to continue to app
- Handle errors with visual feedback

src/server/web/templates/setup.html

@@ -703,10 +703,18 @@
                 const data = await response.json();
 
                 if (response.ok && data.status === 'ok') {
-                    showMessage('Setup completed successfully! Redirecting to login...', 'success');
-                    setTimeout(() => {
-                        window.location.href = '/login';
-                    }, 2000);
+                    // Redirect to loading page if provided, otherwise go to login
+                    if (data.redirect) {
+                        showMessage('Setup saved! Initializing your anime library...', 'success');
+                        setTimeout(() => {
+                            window.location.href = data.redirect;
+                        }, 500);
+                    } else {
+                        showMessage('Setup completed successfully! Redirecting to login...', 'success');
+                        setTimeout(() => {
+                            window.location.href = '/login';
+                        }, 2000);
+                    }
                 } else {
                     const errorMessage = data.detail || data.message || 'Setup failed';
                     showMessage(errorMessage, 'error');