Extract common errorBanner style to shared styles

- Added errorBanner to sharedStyles for consistent error display
- Removed duplicate errorBanner definitions from all pages
- Updated appointments, booking, admin/appointments, and admin/availability pages
- Reduced code duplication while maintaining component-specific styles

frontend/app/admin/availability/page.tsx

@@ -431,16 +431,6 @@ const pageStyles: Record<string, React.CSSProperties> = {
     borderRadius: "8px",
     cursor: "pointer",
   },
-  errorBanner: {
-    fontFamily: "'DM Sans', system-ui, sans-serif",
-    fontSize: "0.9rem",
-    padding: "1rem",
-    background: "rgba(239, 68, 68, 0.15)",
-    border: "1px solid rgba(239, 68, 68, 0.3)",
-    borderRadius: "8px",
-    color: "#f87171",
-    marginBottom: "1rem",
-  },
   calendar: {
     display: "grid",
     gridTemplateColumns: "repeat(auto-fill, minmax(180px, 1fr))",