feat(dashboard): consistent empty states across all pages

[OneStepAt4time]

Motivation

Several dashboard pages show plain text when there's no data. Enterprise UX demands consistent, helpful empty states with icons, descriptions, and CTAs.

Pages to improve

• Overview (no active sessions)
• Session History (no records after filter)
• Pipelines (no pipelines)
• Audit (no audit logs)
• Users (no users)

Approach

• Reusable EmptyState component with icon, title, description, optional CTA button
• Each page passes contextual content
• Consistent styling using design tokens

Acceptance Criteria

• EmptyState component in components/shared/
• All 5+ pages use it
• Accessible (role=status, aria-label)
• Test coverage