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/components/config/JailsTab.tsx

@@ -32,7 +32,7 @@ import {
   Play24Regular,
 } from "@fluentui/react-icons";
 import { ApiError } from "../../api/client";
-import { handleFetchError } from "../../utils/fetchError";
+import { handleFetchError, createStringErrorAdapter } from "../../utils/fetchError";
 import type {
   AddLogPathRequest,
   BackendType,
@@ -733,7 +733,7 @@ function InactiveJailDetail({
     onValidate()
       .then((result) => { setValidationResult(result); })
       .catch((err: unknown) => {
-        handleFetchError(err, setValidationError, "Validation request failed.");
+        handleFetchError(err, createStringErrorAdapter(setValidationError), "Validation request failed.");
       })
       .finally(() => { setValidating(false); });
   }, [onValidate]);