feat: premium API Keys page redesign (OPE-167)

Complete visual overhaul:
- Key cards with tier-colored left accent stripe (amber/indigo/zinc)
- Revoke on hover only, connect guide with tabbed config snippets
- JetBrains Mono key preview, full-width layout
- Empty state with feature hints, emerald key in generate dialog
- No functional changes, same API calls and React Query patterns

Author: DevanshuNEU