feat: recurrence rule editing in EventEditor and ordinal BYDAY support

Add recurrence_frequency, recurrence_interval, recurrence_count, and
recurrence_by_weekday editor fields. Frequency cycles with Space.
BYDAY input accepts comma-separated names and ordinal prefixes (1MO).

Extend RecurrenceRule::by_weekday from std::chrono::weekday to ByWeekday
{ordinal, day} struct. Parser accepts ordinal BYDAY (+1MO, -1FR).
Remove strict BY*/FREQ pairing constraint to match RFC 5545.

tests/calendar_transfer_tests.cpp

@@ -61,7 +61,7 @@ void check_invalid_argument(Function&& function, const std::string_view message)
     recurrence.interval = 2;
     recurrence.count = 6;
     recurrence.until = at(24);
-    recurrence.by_weekday = {Monday, Wednesday};
+    recurrence.by_weekday = {{0, Monday}, {0, Wednesday}};
     recurrence.by_month_day = {1, -1};
     recurrence.by_month = {1, 7};
     event.recurrence = std::move(recurrence);