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/main.py

@@ -26,6 +26,8 @@ from app.infrastructure.di.providers import (
 from app.presentation import router
 from app.presentation.web import auth_router
 from app.presentation.web import router as web_router
+from app.presentation.web.error_handlers import register_error_handlers
+from app.presentation.web.flash import setup_flash_manager
 
 
 @asynccontextmanager
@@ -71,6 +73,14 @@ def app_factory() -> FastAPI:
     setup_dishka(container, app)
 
     register_exception_handlers(app)
+    register_error_handlers(app)
+
+    @app.middleware("http")
+    async def flash_middleware(request, call_next):
+        """Middleware to setup flash manager for each request."""
+        await setup_flash_manager(request)
+        response = await call_next(request)
+        return response
 
     app.add_middleware(
         CORSMiddleware,