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/hooks/usePolledData.ts

@@ -6,6 +6,7 @@
  */
 import { useCallback, useEffect, useRef, useState } from "react";
 import { handleFetchError } from "../utils/fetchError";
+import type { FetchError } from "../types/api";
 
 export interface UsePolledDataOptions<TResponse, TData> {
   fetcher: (signal: AbortSignal) => Promise<TResponse>;
@@ -20,15 +21,23 @@ export interface UsePolledDataOptions<TResponse, TData> {
 export interface UsePolledDataResult<TData> {
   data: TData | null;
   loading: boolean;
-  error: string | null;
+  error: FetchError | null;
   refresh: () => void;
 }
 
 /**
  * Load a single-item response and expose refresh semantics with polling support.
  *
+ * Provides typed error handling through the `error` property, which is either
+ * `null` or a discriminated `FetchError` union. Use the error's `type` field
+ * to determine how to handle it:
+ *
+ * - `"api_error"`: Server returned HTTP error (check `status` for 401/403/50x)
+ * - `"network_error"`: Network, DNS, or JSON parse failure
+ * - `"abort_error"`: Request was cancelled (typically silently ignored by hook)
+ *
  * @param options - Configuration options
- * @returns Data, loading state, error, and refresh callback
+ * @returns Data, loading state, typed error, and refresh callback
  */
 export function usePolledData<TResponse, TData>(
   options: UsePolledDataOptions<TResponse, TData>,
@@ -45,7 +54,7 @@ export function usePolledData<TResponse, TData>(
 
   const [data, setData] = useState<TData | null>(initialData ?? null);
   const [loading, setLoading] = useState(true);
-  const [error, setError] = useState<string | null>(null);
+  const [error, setError] = useState<FetchError | null>(null);
   const abortRef = useRef<AbortController | null>(null);
   const fetchRef = useRef<() => void>((): void => undefined);