feat(dashboard): polish pass — responsive, loading states, error handling

[aegis-gh-agent]

Why

Dashboard panels have been shipped rapidly (#3275, #3276) but haven't had a unified polish pass. Inconsistent loading states, some components lack error handling, responsive behavior needs audit.

What

Full dashboard polish pass across all pages:

1. Loading states — consistent skeleton UI for all panels (pulse animation, correct dimensions)
2. Error states — user-friendly error messages with retry buttons on every data-dependent component
3. Empty states — meaningful illustrations/messages when no data exists (no sessions, no agents, no costs)
4. Responsive — test and fix layout on 768px, 1024px, 1280px, 1920px. Stack charts vertically on narrow screens.
5. Accessibility — audit all interactive elements for keyboard nav, aria labels, focus management
6. Consistent spacing — unify padding/margins across panels using design tokens

Done when

• Every panel has loading, error, and empty states
• Dashboard usable on 1024px+ without horizontal scroll
• axe-core audit passes with zero critical violations
• Visual consistency across all pages

Priority: P2 — polish before enterprise users see it

[OneStepAt4time]

Closed — delivered via PR #3283 (polish pass merged).