feat(frontend): empty/loading/error states + responsive layout

[Omoboi-dev]

Context
Polish that makes the dashboard feel finished and usable on any screen.

Task
Add consistent empty, loading, and error states across views, and make the
layout responsive.

Pointers

• Depends on the scaffold + at least one view existing.
• Consider small shared components for these states.

Acceptance criteria

• Consistent loading/empty/error UI across views
• Layout works on mobile + desktop
• pnpm -r typecheck passes

Notes
🟢 Good first issue.