Add error-state handling and retry UX for API failures

[Dannyswiss1]

client/src/lib/api.ts never fails today, so every page assumes the happy path. Add a way for mock calls to
simulate failure (useful for tests and future real-backend integration), and add error UI to each page: a
"Something went wrong" state with a retry button, plus error toasts for failed mutations (upload, revoke, staff
add, access request).

Acceptance criteria

• Each data-fetching page has a distinct error state, not just infinite/silent loading
• Failed mutations show an error toast (reuse the existing useToast error kind)
• A retry action re-triggers the failed fetch without a full page reload