feat(ui): surface API failures instead of only logging to console

## Priority
Medium

## Why this matters
Several screens catch failures with `console.error`, leaving the user with stale or empty UI and no explanation.

## Evidence
- `packages/ui/src/app/page.tsx` and `packages/ui/src/app/stats/page.tsx` only log fetch errors.
- `packages/ui/src/lib/api/ApiClient.ts` logs raw errors globally.

## Acceptance criteria
- Add reusable loading, empty, and error states.
- Show actionable toast/banner messages for failed mutations and fetches.