feat(filters): add empty and error states

[Fabluchy]

Summary

Adds explicit empty and error states to filters views across events and pairs pages, with retry affordance on error.

Changes

• Events page: Adds filter-empty state ("No events match the filter") when the type filter returns no results but events exist. Enhances error state with a styled alert container and a Retry button.
• Pairs page: Enhances error state with a styled alert container and a Retry button (filter empty states already existed).
• Both error states announce via role="alert" and retry via refetch/api.refetch.
• Filter-empty state lives inside the existing aria-live="polite" region for screen-reader announcements.

Related Issue

Closes #668

Type Of Change

• Feature

Testing

$ npm run lint
✔ No ESLint warnings or errors

$ npm test
Test Suites: 71 passed, 71 total
Tests: 1204 passed, 1204 total

Contributor Checklist

• Branch name follows type/area-slug where possible.
• Tests were added or updated for new UI or behaviour.
• Accessibility was considered (role="alert", aria-live, focus-visible).
• No unrelated CI workflow changes are included.