chore(ui): upgrade frontend dependencies (#1470)

## Summary
- Upgrades all UI dependencies to latest compatible versions
- Resolves all npm audit vulnerabilities (0 remaining)

### Dependencies upgraded
| Package | From | To | Type |
|---------|------|----|------|
| next | 16.1.4 | 16.1.6 | Patch |
| react | 19.2.3 | 19.2.4 | Patch |
| react-dom | 19.2.3 | 19.2.4 | Patch |
| react-hook-form | 7.71.1 | 7.71.2 | Patch |
| lucide-react | 0.562.0 | 0.577.0 | Minor |
| tailwind-merge | 3.4.0 | 3.5.0 | Minor |
| zod | 4.3.5 | 4.3.6 | Patch |
| zustand | 5.0.10 | 5.0.11 | Patch |
| @a2a-js/sdk | 0.3.8 | 0.3.10 | Patch |
| @types/node | 25.0.9 | 25.4.0 | Minor |
| @types/react | 19.2.9 | 19.2.14 | Patch |
| eslint-config-next | 16.1.4 | 16.1.6 | Patch |
| jest | 30.2.0 | 30.3.0 | Minor |
| cypress | 15.9.0 | 15.11.0 | Minor |
| + other dev deps (msw, postcss, autoprefixer, etc.) | | | |

### Skipped (major version, require migration)
- **eslint** 9.x → 10.x
- **tailwindcss** 3.x → 4.x

## Test plan
- [x] `npm run lint` passes
- [x] `npm run build` succeeds
- [x] `npm test` — all 107 tests pass
- [ ] CI pipeline passes

Signed-off-by: Jaison Paul <paul.jaison@gmail.com>

Author: jsonmp-k8