Add multi-day and recurring calendar events

[darshan-Jahagirdar]

Fixes #377.

This adds calendar creation support for:

• multi-day events by expanding the selected date range into normal calendar rows
• daily, weekly, and monthly recurring events up to a chosen end date
• recurring multi-day spans by preserving the span duration for each occurrence

The implementation avoids a database migration by storing generated occurrences as existing fcms_calendar rows, so the current month view, day view, homepage widgets, and exports continue to read the same table shape.

Verification:

• git diff --check -- familyconnections/calendar.php familyconnections/inc/calendar_class.php familyconnections/ui/themes/default/templates/calendar/add.php passes locally.
• Codacy Static Code Analysis is passing with 0 new issues.
• Could not run PHP locally because this machine does not have php, Composer, or Docker installed on PATH.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 321 complexity · 0 duplication

Metric	Results
Complexity	321
Duplication	0

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