feat: Implement typed error contracts in generic hooks

Introduce discriminated FetchError union type to replace weak string error
handling in API calls and hooks. Enables actionable error diagnostics.

Changes:
- Create types/api.ts with FetchError discriminated union (api_error,
  network_error, abort_error)
- Export type guards: isAuthError, isAbortError, isNetworkError, isApiError
- Update useListData and usePolledData to expose typed FetchError instead of
  string
- Add getErrorMessage() helper to extract displayable messages from FetchError
- Add createStringErrorAdapter() for backward compatibility with string error
  state
- Update handleFetchError() to work with both FetchError and string setters
- Update all consumer hooks to expose typed errors
- Update components to use getErrorMessage() when displaying errors
- Update tests to mock FetchError instead of strings
- Add comprehensive typed error model documentation to Web-Development.md

This enables better error handling patterns:
- Check error.type to distinguish between API, network, and abort errors
- Extract status codes for specific handling (401/403 auth, 50x server errors)
- Maintain backward compatibility with existing string-based error states

All TypeScript compilation passes with no errors.

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

frontend/src/types/api.ts

@@ -0,0 +1,106 @@
+/**
+ * Typed API error contracts and models.
+ *
+ * Provides discriminated error types for standardized error handling across
+ * API calls and hooks, enabling actionable error reporting and diagnostics.
+ */
+
+/**
+ * Discriminated error type representing an HTTP API error response.
+ *
+ * Thrown when the server returns a non-2xx HTTP status code.
+ * Use the `type` discriminator to handle different error categories.
+ */
+export interface ApiErrorPayload {
+  type: "api_error";
+  /** HTTP status code returned by the server. */
+  status: number;
+  /** Raw response body text as returned by the server. */
+  body: string;
+  /** User-friendly error message derived from status and body. */
+  message: string;
+}
+
+/**
+ * Discriminated error type representing a network error.
+ *
+ * Thrown when the request fails due to network issues (DNS lookup failure,
+ * connection timeout, offline, CORS error, etc.) or when parsing JSON fails.
+ */
+export interface NetworkErrorPayload {
+  type: "network_error";
+  /** Underlying error message (network stack or JSON parse error). */
+  message: string;
+}
+
+/**
+ * Discriminated error type representing a request abort.
+ *
+ * Thrown when a fetch request is aborted (e.g., component unmounts, user cancels).
+ * These are expected and should typically be silently ignored.
+ */
+export interface AbortErrorPayload {
+  type: "abort_error";
+  message: string;
+}
+
+/**
+ * Union of all possible fetch error types.
+ *
+ * Use the `type` discriminator to narrow and handle each error case:
+ *
+ * ```ts
+ * const error: FetchError = ...;
+ * if (error.type === "api_error") {
+ *   // Handle HTTP error — check status for 401/403/50x etc.
+ *   if (error.status === 401) redirectToLogin();
+ * } else if (error.type === "network_error") {
+ *   // Handle network/connectivity issues
+ *   showNetworkMessage();
+ * } else if (error.type === "abort_error") {
+ *   // Request was cancelled — typically silent
+ *   return;
+ * }
+ * ```
+ */
+export type FetchError = ApiErrorPayload | NetworkErrorPayload | AbortErrorPayload;
+
+/**
+ * Type guard to check if an error is an authentication error (401/403).
+ *
+ * @param error - The error to check (typically a FetchError)
+ * @returns `true` if the error is a 401 or 403 API error
+ */
+export function isAuthError(error: FetchError): error is ApiErrorPayload {
+  return error.type === "api_error" && (error.status === 401 || error.status === 403);
+}
+
+/**
+ * Type guard to check if an error is an abort error (request cancelled).
+ *
+ * @param error - The error to check (typically a FetchError)
+ * @returns `true` if the error is an abort error
+ */
+export function isAbortError(error: FetchError): error is AbortErrorPayload {
+  return error.type === "abort_error";
+}
+
+/**
+ * Type guard to check if an error is a network error.
+ *
+ * @param error - The error to check (typically a FetchError)
+ * @returns `true` if the error is a network error
+ */
+export function isNetworkError(error: FetchError): error is NetworkErrorPayload {
+  return error.type === "network_error";
+}
+
+/**
+ * Type guard to check if an error is an API error.
+ *
+ * @param error - The error to check (typically a FetchError)
+ * @returns `true` if the error is an API error
+ */
+export function isApiError(error: FetchError): error is ApiErrorPayload {
+  return error.type === "api_error";
+}