Replace any type with proper ical-generator types for type safety

[Copilot]

Addresses type safety feedback on PR #1 regarding the use of any type for the recurrence rule object in lib/ical-generator.ts.

Changes:

• Import ICalRepeatingOptions and ICalEventRepeatingFreq from ical-generator
• Replace any type annotation with ICalRepeatingOptions for rrule variable
• Use ICalEventRepeatingFreq.WEEKLY enum instead of string literal 'WEEKLY'

Before:

let rrule: any = {
  freq: 'WEEKLY',
  count: 40
}

After:

const rrule: ICalRepeatingOptions = {
  freq: ICalEventRepeatingFreq.WEEKLY,
  count: 40
}

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