Add meeting join buttons and current/next event highlighting

[DanielLaberge]

Summary

• Add Join Meeting buttons in the agenda and compact join controls in horizontal/vertical bars, using the default URL handler.
• Highlight the current timed event, or the next event when none is active; skip all-day and cancelled events.
• Keep event-detail clicks separate from meeting-link clicks.

Validation

Both bash tests/test-next-event.sh and node tests/test-widget-customizations.js pass. Rendered the actual widget in Quickshell with fictional demo events for the screenshots below.

Current meeting

Next meeting

[DanielLaberge]

If you would prefer this be enabled through a toggle in the settings UI, or if you'd like this PR to also update the main README.md, let me know.

Cheers.