feat(ui): add error handling, flash messages and SEO optimization

- Add custom error pages (404, 403, 500) with user-friendly messages
- Add flash message system with signed cookies for security
- Add toast notifications with auto-dismiss and manual close
- Add comprehensive SEO meta tags (description, keywords, OG, Twitter)
- Add canonical URLs for SEO
- Update routes to use slug-based URLs (/posts/{slug} instead of /posts/{id})
- Add Open Graph and Twitter Card meta tags for social sharing
- Add favicon SVG
- Update all templates with proper meta tags and URLs
- Add error handlers registration in main.py
- Add flash middleware for request handling
- Install itsdangerous dependency

app/presentation/web/__init__.py

@@ -9,6 +9,7 @@ be integrated with use cases in future iterations.
 """
 
 from app.presentation.web.auth import router as auth_router
+from app.presentation.web.error_handlers import register_error_handlers
 from app.presentation.web.routes import router
 
-__all__ = ["router", "auth_router"]
+__all__ = ["router", "auth_router", "register_error_handlers"]