Migrate to KnowAll standard tech stack

## Summary
Quick migration to align with standard stack (already very close).

## Tasks
- [ ] Switch pnpm → Bun
- [ ] Update Tailwind 3.4 → 4
- [ ] Update ESLint 8 → 9 (flat config)
- [ ] Add Chart.js for any future reporting needs

## Current Stack
- Next.js 15.5 ✓
- shadcn/ui ✓
- Tailwind 3.4 → 4
- ESLint 8 → 9
- pnpm → Bun

## Effort: Low