feat: virtualize timeline rendering

[yahorbarkouski]

Summary

• flatten date-grouped timeline data into virtual header/row items and render only visible rows with @tanstack/react-virtual
• keep the existing pagination controls while making grid columns responsive via ResizeObserver
• add a focused test for the virtual timeline item builder and expose the scroll viewport ref for virtualization

Closes #7

Verification

• ./node_modules/.bin/biome check src/components/ui/scroll-area.tsx src/components/timeline/Timeline.tsx src/components/timeline/TimelineGroup.tsx src/components/timeline/useResponsiveColumns.ts src/components/timeline/virtualTimeline.ts electron/main/features/__tests__/virtualTimeline.test.ts
• npm run typecheck
• ./node_modules/.bin/vitest run electron/main/features/__tests__/virtualTimeline.test.ts

Notes

• npm run lint still fails on a pre-existing unrelated file: electron/main/features/mobileActivity/LocalMobileBridgeService.ts

[github-actions]

🎉 This PR is included in version 1.30.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀