feat: centralized error notification service (issue #15)

- Create NotificationService with context provider for centralized error/success messaging
- Add NotificationContainer component to render notification stack
- Integrate NotificationProvider into App root
- Refactor BanUnbanForm to use notification service instead of local error state
- Update fetchError utility to optionally use notification callbacks
- Add comprehensive error handling guidelines to Web-Development.md
- Prevent duplicate notifications with deduplication logic
- Support auto-dismiss with configurable TTL per notification type

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/utils/fetchError.ts

@@ -2,19 +2,40 @@ import { isAuthError } from "../api/client";
 
 /**
  * Normalize fetch error handling across hooks.
+ *
+ * Handles three error cases:
+ * 1. Request was aborted — silently ignored (expected cleanup)
+ * 2. Auth error (401/403) — silently handled by AuthProvider (do not display)
+ * 3. Other error — stored in component state or notified via callback
+ *
+ * @param err - The caught error
+ * @param setError - State setter to store error message (used when no notification callback)
+ * @param fallback - Default error message if err is not an Error instance
+ * @param onError - Optional callback to notify of errors instead of using setError
  */
 export function handleFetchError(
   err: unknown,
   setError: (value: string | null) => void,
   fallback: string = "Unknown error",
+  onError?: (message: string) => void,
 ): void {
+  // Abort errors are expected during cleanup — ignore silently
   if (err instanceof DOMException && err.name === "AbortError") {
     return;
   }
 
+  // Auth errors are handled globally by AuthProvider — do not display locally
   if (isAuthError(err)) {
     return;
   }
 
-  setError(err instanceof Error ? err.message : fallback);
+  const message = err instanceof Error ? err.message : fallback;
+
+  // Use notification callback if provided; otherwise use local state setter
+  if (onError) {
+    onError(message);
+  } else {
+    setError(message);
+  }
 }
+