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/types/notification.ts

@@ -0,0 +1,53 @@
+/**
+ * Notification type definitions.
+ * Defines the contract for error, success, warning, and info messages
+ * rendered through the centralized notification service.
+ */
+
+/** Severity level of a notification. */
+export type NotificationIntent = "error" | "success" | "warning" | "info";
+
+/** Unique notification message. */
+export interface Notification {
+  /** Unique identifier for this notification (auto-generated). */
+  id: string;
+  /** Severity level. */
+  intent: NotificationIntent;
+  /** User-facing message text. */
+  message: string;
+  /** Auto-dismiss after this many milliseconds. Null means manual dismiss only. */
+  autoCloseMsec?: number | null;
+}
+
+/** Notification service public API. */
+export interface NotificationService {
+  /**
+   * Show a success notification.
+   * @param message User-facing message text.
+   * @param autoCloseMsec Auto-dismiss after this many ms. Defaults to 5000.
+   */
+  success(message: string, autoCloseMsec?: number): void;
+  /**
+   * Show an error notification.
+   * @param message User-facing message text.
+   * @param autoCloseMsec Auto-dismiss after this many ms. Defaults to 8000.
+   */
+  error(message: string, autoCloseMsec?: number): void;
+  /**
+   * Show a warning notification.
+   * @param message User-facing message text.
+   * @param autoCloseMsec Auto-dismiss after this many ms. Defaults to 6000.
+   */
+  warning(message: string, autoCloseMsec?: number): void;
+  /**
+   * Show an info notification.
+   * @param message User-facing message text.
+   * @param autoCloseMsec Auto-dismiss after this many ms. Defaults to 5000.
+   */
+  info(message: string, autoCloseMsec?: number): void;
+  /**
+   * Remove a notification by ID.
+   * Called automatically on auto-dismiss; can also be called manually.
+   */
+  dismiss(id: string): void;
+}