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/flash.py

@@ -0,0 +1,160 @@
+"""Flash messages middleware for web UI.
+
+This module provides flash message functionality for the web interface,
+allowing temporary messages to be passed between requests (e.g., after redirect).
+Uses signed cookies for security.
+"""
+
+from fastapi import Request, Response
+from itsdangerous import URLSafeSerializer
+
+from app.infrastructure.config.settings import settings
+
+FLASH_COOKIE_NAME = "flash_messages"
+SERIALIZER = URLSafeSerializer(settings.security.secret_key.get_secret_value())  # type: ignore[union-attr]
+
+
+class FlashMessage:
+    """Flash message model.
+
+    Represents a single flash message with type and content.
+
+    Attributes:
+        message: The message text.
+        category: Message category (success, error, warning, info).
+    """
+
+    def __init__(self, message: str, category: str = "info") -> None:
+        """Initialize flash message.
+
+        Args:
+            message: The message text.
+            category: Message category (success, error, warning, info).
+        """
+        self.message = message
+        self.category = category
+
+    def to_dict(self) -> dict[str, str]:
+        """Convert to dictionary.
+
+        Returns:
+            Dictionary with message and category.
+        """
+        return {"message": self.message, "category": self.category}
+
+
+class FlashManager:
+    """Manager for flash messages.
+
+    Handles storing and retrieving flash messages from cookies.
+    Messages are cleared after being read.
+
+    Attributes:
+        request: FastAPI request object.
+        messages: List of current messages.
+    """
+
+    CATEGORIES = {
+        "success": "success",
+        "error": "error",
+        "warning": "warning",
+        "info": "info",
+    }
+
+    def __init__(self, request: Request) -> None:
+        """Initialize flash manager.
+
+        Args:
+            request: FastAPI request object.
+        """
+        self.request = request
+        self.messages: list[FlashMessage] = []
+        self._load_messages()
+
+    def _load_messages(self) -> None:
+        """Load messages from cookie."""
+        cookie_value = self.request.cookies.get(FLASH_COOKIE_NAME)
+        if cookie_value:
+            try:
+                data = SERIALIZER.loads(cookie_value)
+                if isinstance(data, list):
+                    self.messages = [FlashMessage(msg["message"], msg["category"]) for msg in data]
+            except Exception:
+                self.messages = []
+
+    def add(self, message: str, category: str = "info") -> None:
+        """Add a flash message.
+
+        Args:
+            message: The message text.
+            category: Message category (success, error, warning, info).
+        """
+        self.messages.append(FlashMessage(message, category))
+
+    def get_messages(self) -> list[dict[str, str]]:
+        """Get all messages and clear them.
+
+        Returns:
+            List of message dictionaries.
+        """
+        result = [msg.to_dict() for msg in self.messages]
+        self.messages = []
+        return result
+
+    def has_messages(self) -> bool:
+        """Check if there are any messages.
+
+        Returns:
+            True if there are messages.
+        """
+        return len(self.messages) > 0
+
+    def set_cookie(self, response: Response) -> None:
+        """Set flash cookie on response.
+
+        Args:
+            response: FastAPI response object.
+        """
+        if self.messages:
+            data = [msg.to_dict() for msg in self.messages]
+            cookie_value = SERIALIZER.dumps(data)
+            response.set_cookie(
+                key=FLASH_COOKIE_NAME,
+                value=cookie_value,
+                httponly=True,
+                secure=not settings.is_dev,
+                samesite="lax",
+                max_age=300,  # 5 minutes
+            )
+        else:
+            response.delete_cookie(key=FLASH_COOKIE_NAME)
+
+
+def flash(request: Request, message: str, category: str = "info") -> None:
+    """Add flash message to request state.
+
+    Convenience function to add flash message.
+    Must be called before response is created.
+
+    Args:
+        request: FastAPI request object.
+        message: The message text.
+        category: Message category.
+    """
+    if not hasattr(request.state, "flash_manager"):
+        request.state.flash_manager = FlashManager(request)
+    request.state.flash_manager.add(message, category)
+
+
+def get_flash_messages(request: Request) -> list[dict[str, str]]:
+    """Get flash messages from request.
+
+    Args:
+        request: FastAPI request object.
+
+    Returns:
+        List of flash message dictionaries.
+    """
+    if hasattr(request.state, "flash_manager"):
+        return request.state.flash_manager.get_messages()
+    return []