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/controllers/page_controller.py

@@ -49,3 +49,13 @@ async def queue_page(request: Request):
         request,
         title="Download Queue - Aniworld"
     )
+
+
+@router.get("/loading", response_class=HTMLResponse)
+async def loading_page(request: Request):
+    """Serve the initialization loading page."""
+    return render_template(
+        "loading.html",
+        request,
+        title="Initializing - Aniworld"
+    )