feat: add event end date and format date range

[jdorfman]

• Add endDate field to Event interface
• Implement formatDateRange function to handle single-day and multi-day events
• Update event date display to use formatDateRange

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[jdorfman]

@chris-sev, this has been on my to-do list for a while. Most events are multi-day, and sometimes, they start in one month and go on to the next.

https://deploy-preview-7439--sourcegraph.netlify.app/events

[jdorfman]

Good call, looks way better with the space