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/useJailList.ts

@@ -13,12 +13,13 @@ import {
 } from "../api/jails";
 import { useListData } from "./useListData";
 import type { JailSummary, JailListResponse } from "../types/jail";
+import type { FetchError } from "../types/api";
 
 export interface UseJailsResult {
   jails: JailSummary[];
   total: number;
   loading: boolean;
-  error: string | null;
+  error: FetchError | null;
   refresh: () => void;
   startJail: (name: string) => Promise<void>;
   stopJail: (name: string) => Promise<void>;