fix: correct recurring-event expansion in Calendar

[raihanmd]

Replace the 28-day MONTHLY step (addWeeks(current,4)) with addMonths(current,1) so month-end anchors keep their day-of-month, and bound the expansion loop by windowEnd instead of a fixed 52-iteration cap so DAILY fills the full ~26-week window.

Description

Fixes #1046 Calendar.expandRecurring produced wrong instances for recurring events.

• MONTHLY drift: the step used addWeeks(current, 4) (a fixed 28 days), so a monthly event anchored on, e.g., Jan 31 landed on Feb 28 / Mar 31 instead of the same day each month. It now steps with addMonths(current, 1), preserving the day-of-month (month-end dates clamp correctly, e.g. Jan 31 -> Feb 28).
• DAILY truncation: the loop was capped at a fixed i < 52 iterations, which only yielded ~7 weeks for daily events. The loop is now bounded by windowEnd (addWeeks(new Date(), 26)), so daily events fill the full intended ~26-week window.
  No new dependency addMonths already ships in date-fns v3. WEEKLY and non-recurring behavior are unchanged.
  A focused unit test (src/testing/events/recurrence.test.ts) covers WEEKLY (7-day steps), DAILY (full window, 1-day steps), MONTHLY (real calendar months via addMonths), and non-recurring passthrough. pnpm run type-check and pnpm run lint pass; the new test passes 5/5.

Related Issue

Closes #1046

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• Code follows project style guidelines
• Self-review completed
• No console errors
• Uses Lucide icons consistently (no new icons added; existing usage unchanged)
• Responsive design implemented (logic-only change; no new UI/layout)
• Starknet best practices followed; N/A: teachLink is not a Starknet project (template artifact, left unchecked)

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