fix(ui): show exact timestamps for change dates

[tracerat]

Summary

• replace the shared relative “about X days ago” date helper with an exact UTC timestamp formatter
• update registry, workflow version, admin, organization, and watchtower timestamp displays to use exact UTC values
• add unit coverage for exact timestamp formatting

Testing

• pnpm --dir frontend exec biome check src/lib/event-history.ts tests/event-history.test.ts src/components/watchtower/watchtower-monitor.tsx src/components/registry/dialogs/repository-commit-dialog.tsx src/components/registry/dialogs/repository-versions-dialog.tsx src/components/registry/commit-selector.tsx src/components/builder/panel/workflow-panel.tsx src/components/organization/workflow-pull-dialog.tsx src/components/organization/org-sessions-table.tsx src/components/organization/org-members-table.tsx src/components/agents/agent-preset-versions-panel.tsx src/components/admin/platform-registry-repos-table.tsx src/components/admin/platform-registry-status.tsx src/components/admin/admin-org-registry-table.tsx src/components/admin/org-registry-versions-panel.tsx src/components/admin/admin-users-table.tsx src/components/admin/platform-registry-versions-table.tsx tests/agent-preset-versions-panel.test.tsx
• pnpm --dir frontend test -- event-history.test.ts agent-preset-versions-panel.test.tsx
• pnpm --dir frontend typecheck

---

Summary by cubic

Show exact UTC timestamps instead of “about X ago” for change dates across the app to improve clarity and match logs. Adds a shared formatter and updates displays in registry, workflows, admin, org, and watchtower.

• Bug Fixes
  • Introduced formatExactTimestamp() in event-history for exact UTC output and invalid-date handling.
  • Replaced relative time rendering in admin tables, registry dialogs/selectors, workflow/version views, org members/sessions, and watchtower.
  • Added unit tests and updated mocks to cover the new formatter.

^{Written for commit d2135d9. Summary will update on new commits.}