feat(a11y): add automated axe checks for Dashboard, Bridges, Watchlist

[starfishkrq]

Closes #27

• Wire vitest-axe toHaveNoViolations matcher into shared test setup (src/test/setup.ts)
• Add TypeScript augmentation for toHaveNoViolations across the suite (src/test/vitest-axe.d.ts)
• Add axe assertions for the four highest-traffic pages:
  • src/pages/Dashboard.a11y.test.tsx — mocks all heavy sub-components/hooks, 15s timeout for complex page scan
  • src/pages/Bridges.a11y.test.tsx — mocks hooks and store selectors, verifies rendered bridge grid
  • src/pages/Watchlist.a11y.test.tsx — covers empty state and populated state
  • src/pages/Alerts.a11y.test.tsx — covers loaded and loading states via useIncidentFeed mock
• Fix axe violations surfaced on first pass:
  • src/components/alerts/CompactAlertList.tsx: promote role=table to role=grid, add role=columnheader/gridcell to all header/data cells, add aria-sort to column headers, add aria-label to sort buttons, mark decorative SVG arrows aria-hidden
  • src/components/BridgeStatusCard.tsx: replace h3 with p to remove heading-order violation inside card
• All four pages pass toHaveNoViolations with no suppressions
• Tests run automatically under the existing Unit tests CI step