Replace custom Repetition with RRule

[firu11]

As preparation for full iCal compatibility, I migrated our custom Repetition struct to the battle-tested rrule-go library. iCal uses the powerful RRule standard for recurrences, including complex weekday rules.

This change:

• Adds rich repetition capabilities (e.g. “every other week on Monday and Friday”)
• Simplifies the core package by removing custom recurrence logic
• Enables much cleaner iCal integration