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/templates/pages/post_form.html

@@ -9,9 +9,9 @@
     </h1>
 </section>
 
-<form 
-    method="POST" 
-    action="{% if is_edit %}/posts/{{ post.id }}/edit{% else %}/posts/new{% endif %}"
+<form
+    method="POST"
+    action="{% if is_edit %}/web/posts/{{ post.slug.value }}/edit{% else %}/web/posts/new{% endif %}"
     class="card"
     data-testid="form-post"
 >
@@ -81,7 +81,7 @@
     
     <div class="card-footer" data-testid="form-post-footer">
         <div class="flex justify-between items-center" data-testid="form-actions">
-            <a href="{% if is_edit %}/posts/{{ post.id }}{% else %}/{% endif %}" class="btn" data-testid="btn-cancel">
+            <a href="{% if is_edit %}/web/posts/{{ post.slug.value }}{% else %}/web/{% endif %}" class="btn" data-testid="btn-cancel">
                 Cancel
             </a>