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>

Docs/Web-Development.md

@@ -810,10 +810,123 @@ When an API request returns 401 or 403:
 
 ## 12. Error Handling & Resilience
 
+### Centralized Error Notification Service
+
+The application provides a centralized `NotificationService` for displaying consistent, user-facing error and success messages across the entire UI. This prevents fragmented error handling and duplicate messaging.
+
+**When to use:**
+- **Form operations** (ban, unban, save settings) → use global notifications for success/error feedback
+- **Async actions** that affect application state → use notifications to inform the user
+- **API errors** in components → convert to user-friendly messages and show via notifications
+
+**When NOT to use (keep local state):**
+- Input validation errors that should not trigger API calls (display inline field errors instead)
+- Component-internal loading states (no need for notifications)
+- Errors in error boundaries (error boundaries have their own fallback UI)
+
+**API:**
+
+```ts
+// In any component or hook within NotificationProvider
+const notification = useNotification();
+
+notification.success("Data saved successfully!");
+notification.error("Failed to save: invalid format");
+notification.warning("This action cannot be undone");
+notification.info("Processing in background…");
+
+// Control auto-dismiss duration (milliseconds)
+notification.error("Connection lost", 10000); // Auto-dismiss after 10 seconds
+notification.error("Critical error", null); // Never auto-dismiss
+```
+
+**Auto-dismiss defaults:**
+- Success: 5000ms
+- Error: 8000ms
+- Warning: 6000ms
+- Info: 5000ms
+
+**Duplicate prevention:** The service prevents identical notifications from appearing multiple times. If you try to show the same message with the same intent level twice, the second is ignored.
+
+**Example refactor — from local state to notifications:**
+
+```ts
+// Before: Local error/success state in component
+function MyForm() {
+  const [error, setError] = useState<string | null>(null);
+  const [success, setSuccess] = useState<string | null>(null);
+
+  const handleSubmit = async () => {
+    setError(null);
+    setSuccess(null);
+    try {
+      await saveData();
+      setSuccess("Data saved!");
+    } catch (err) {
+      setError("Failed to save");
+    }
+  };
+
+  return (
+    <div>
+      {error && <MessageBar intent="error">{error}</MessageBar>}
+      {success && <MessageBar intent="success">{success}</MessageBar>}
+      <button onClick={handleSubmit}>Save</button>
+    </div>
+  );
+}
+
+// After: Using notification service
+function MyForm() {
+  const notification = useNotification();
+
+  const handleSubmit = async () => {
+    try {
+      await saveData();
+      notification.success("Data saved!");
+    } catch (err) {
+      notification.error("Failed to save");
+    }
+  };
+
+  return <button onClick={handleSubmit}>Save</button>;
+}
+```
+
+**Using notifications with hooks:**
+
+If a hook manages data fetching and needs to notify of errors, accept a notification callback or use `useNotification` directly within the hook:
+
+```ts
+// hooks/useSaveData.ts
+export function useSaveData() {
+  const notification = useNotification();
+  const [loading, setLoading] = useState(false);
+
+  const save = useCallback(async (data: unknown) => {
+    setLoading(true);
+    try {
+      await api.post("/data", data);
+      notification.success("Data saved");
+      return true;
+    } catch (err) {
+      notification.error("Failed to save data");
+      return false;
+    } finally {
+      setLoading(false);
+    }
+  }, [notification]);
+
+  return { save, loading };
+}
+```
+
 ### API Error Handling
 
 - Wrap API calls in `try-catch` inside hooks — components should never see raw exceptions.
-- **All hook catch blocks must use `handleFetchError` rather than directly calling `setError`.** This ensures auth errors (401/403) are routed to the global session-expiry flow instead of displaying confusing error text in the UI. Use the pattern: `handleFetchError(err, setError, "User-friendly fallback message")`.
+- **All hook catch blocks should use `handleFetchError` or notifications.** This ensures auth errors (401/403) are routed to the global session-expiry flow instead of displaying confusing error text in the UI.
+  - **With local state:** `handleFetchError(err, setError, "User-friendly fallback message")`
+  - **With notifications:** `handleFetchError(err, setError, "Default message", notification.error)`
 - Display user-friendly error messages — never expose stack traces or raw server responses in the UI.
 - Log errors to the console (or a future logging service) with sufficient context for debugging.
 - Always handle the **loading**, **error**, and **empty** states for every data-driven component.