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/error_handlers.py

@@ -0,0 +1,188 @@
+"""Error handlers and middleware for web UI.
+
+This module provides custom error pages and flash message middleware
+for the web interface.
+"""
+
+from typing import Any
+
+from fastapi import HTTPException, Request
+from fastapi.responses import HTMLResponse, RedirectResponse
+from fastapi.templating import Jinja2Templates
+
+from app.presentation.web.flash import FlashManager, get_flash_messages
+
+templates = Jinja2Templates(directory="app/presentation/templates")
+
+
+async def setup_flash_manager(request: Request) -> None:
+    """Setup flash manager on request state.
+
+    Args:
+        request: FastAPI request object.
+    """
+    request.state.flash_manager = FlashManager(request)
+
+
+async def add_flash_to_response(request: Request, response: HTMLResponse) -> None:
+    """Add flash cookie to response if needed.
+
+    Args:
+        request: FastAPI request object.
+        response: FastAPI response object.
+    """
+    if hasattr(request.state, "flash_manager"):
+        request.state.flash_manager.set_cookie(response)
+
+
+def get_template_context(request: Request) -> dict[str, Any]:
+    """Get base template context with flash messages.
+
+    Args:
+        request: FastAPI request object.
+
+    Returns:
+        Template context dictionary.
+    """
+    from app.presentation.web.deps import can_create_post, get_user_role
+
+    user = getattr(request.state, "user", None)
+    user_role = get_user_role(user)
+
+    return {
+        "request": request,
+        "user": user,
+        "user_role": user_role.value if user_role else None,
+        "can_create": can_create_post(user),
+        "flash_messages": get_flash_messages(request),
+    }
+
+
+async def http_exception_handler(request: Request, exc: HTTPException) -> HTMLResponse:
+    """Handle HTTP exceptions with custom error pages.
+
+    Args:
+        request: FastAPI request object.
+        exc: HTTPException instance.
+
+    Returns:
+        HTMLResponse with error page.
+    """
+    # Handle redirects (307, 308)
+    if exc.status_code in (307, 308) and "Location" in exc.headers:
+        return RedirectResponse(url=exc.headers["Location"], status_code=exc.status_code)
+
+    error_pages = {
+        403: ("Access Denied", "You don't have permission to access this page."),
+        404: ("Page Not Found", "The page you're looking for doesn't exist."),
+        500: ("Server Error", "Something went wrong on our end. Please try again later."),
+    }
+
+    error_title, error_message = error_pages.get(
+        exc.status_code, ("Error", exc.detail or "An unexpected error occurred.")
+    )
+
+    context = get_template_context(request)
+    context.update(
+        {
+            "error_code": exc.status_code,
+            "error_title": error_title,
+            "error_message": error_message,
+        }
+    )
+
+    return templates.TemplateResponse(
+        "pages/error.html",
+        context,
+        status_code=exc.status_code,
+    )
+
+
+async def not_found_handler(request: Request, exc: HTTPException) -> HTMLResponse:
+    """Handle 404 Not Found errors.
+
+    Args:
+        request: FastAPI request object.
+        exc: HTTPException instance.
+
+    Returns:
+        HTMLResponse with 404 page.
+    """
+    context = get_template_context(request)
+    context.update(
+        {
+            "error_code": 404,
+            "error_title": "Page Not Found",
+            "error_message": "The page you're looking for doesn't exist or has been moved.",
+        }
+    )
+
+    return templates.TemplateResponse(
+        "pages/error.html",
+        context,
+        status_code=404,
+    )
+
+
+async def forbidden_handler(request: Request, exc: HTTPException) -> HTMLResponse:
+    """Handle 403 Forbidden errors.
+
+    Args:
+        request: FastAPI request object.
+        exc: HTTPException instance.
+
+    Returns:
+        HTMLResponse with 403 page.
+    """
+    context = get_template_context(request)
+    context.update(
+        {
+            "error_code": 403,
+            "error_title": "Access Denied",
+            "error_message": "You don't have permission to access this resource. Please sign in or contact an administrator.",
+        }
+    )
+
+    return templates.TemplateResponse(
+        "pages/error.html",
+        context,
+        status_code=403,
+    )
+
+
+async def server_error_handler(request: Request, exc: Exception) -> HTMLResponse:
+    """Handle 500 Internal Server Error.
+
+    Args:
+        request: FastAPI request object.
+        exc: Exception instance.
+
+    Returns:
+        HTMLResponse with 500 page.
+    """
+    context = get_template_context(request)
+    context.update(
+        {
+            "error_code": 500,
+            "error_title": "Server Error",
+            "error_message": "Something went wrong on our end. Please try again later or contact support if the problem persists.",
+        }
+    )
+
+    return templates.TemplateResponse(
+        "pages/error.html",
+        context,
+        status_code=500,
+    )
+
+
+def register_error_handlers(app) -> None:
+    """Register error handlers with FastAPI app.
+
+    Args:
+        app: FastAPI application instance.
+    """
+    app.add_exception_handler(404, not_found_handler)
+    app.add_exception_handler(403, forbidden_handler)
+    app.add_exception_handler(500, server_error_handler)
+    app.add_exception_handler(HTTPException, http_exception_handler)