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

pyproject.toml

@@ -15,6 +15,7 @@ dependencies = [
     "dishka>=1.5.0",
     "httpx>=0.28.0",
     "jinja2>=3.1.6",
+    "itsdangerous>=2.2.0",
 ]
 
 [build-system]