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/useTimezoneData.ts

@@ -1,6 +1,6 @@
 import { useCallback, useEffect, useState } from "react";
 import { fetchTimezone } from "../api/setup";
-import { handleFetchError } from "../utils/fetchError";
+import { handleFetchError, createStringErrorAdapter } from "../utils/fetchError";
 
 export interface UseTimezoneDataResult {
   timezone: string;
@@ -24,7 +24,7 @@ export function useTimezoneData(): UseTimezoneDataResult {
       setTimezone(resp.timezone);
     } catch (err: unknown) {
       if (signal?.aborted) return;
-      handleFetchError(err, setError, "Failed to fetch timezone");
+      handleFetchError(err, createStringErrorAdapter(setError), "Failed to fetch timezone");
       setTimezone("UTC");
     } finally {
       if (!signal?.aborted) {