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/useJailAdmin.ts

@@ -10,7 +10,7 @@ import {
   validateJailConfig,
   createJailConfigFile,
 } from "../api/config";
-import { handleFetchError } from "../utils/fetchError";
+import { handleFetchError, createStringErrorAdapter } from "../utils/fetchError";
 import type {
   ActivateJailRequest,
   ConfFileCreateRequest,
@@ -57,7 +57,7 @@ export function useJailAdmin(): UseJailAdminResult {
       })
       .catch((err: unknown) => {
         if (!ctrl.signal.aborted) {
-          handleFetchError(err, setInactiveError, "Failed to load inactive jails");
+          handleFetchError(err, createStringErrorAdapter(setInactiveError), "Failed to load inactive jails");
         }
       })
       .finally(() => {