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.

include/nocal/domain/event.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <chrono>
+#include <cstdint>
 #include <optional>
 #include <string>
 #include <vector>
@@ -14,12 +15,20 @@ enum class TimeBasis { floating, utc, zoned };
 
 enum class RecurrenceFrequency { daily, weekly, monthly, yearly };
 
+// BYDAY selector. For non-ordinal: use {0, weekday}. For ordinal: {n, weekday}.
+struct ByWeekday {
+    int ordinal{0};  // 0 = non-ordinal; 1-5 or -5 to -1 = ordinal position
+    std::chrono::weekday day;
+
+    bool operator==(const ByWeekday&) const = default;
+};
+
 struct RecurrenceRule {
     RecurrenceFrequency frequency{RecurrenceFrequency::daily};
     unsigned interval{1};
     std::optional<unsigned> count;
     std::optional<TimePoint> until;
-    std::vector<std::chrono::weekday> by_weekday;
+    std::vector<ByWeekday> by_weekday;
     std::vector<int> by_month_day;
     std::vector<unsigned> by_month;
 

include/nocal/tui/EventEditor.hpp

@@ -25,6 +25,11 @@ enum class EditorField : std::size_t {
     all_day,
     location,
     notes,
+    // Recurrence fields
+    recurrence_frequency,  // none/daily/weekly/monthly/yearly
+    recurrence_interval,   // number
+    recurrence_count,      // number or empty for unlimited
+    recurrence_by_weekday, // comma-separated: MO,TU,WE,TH,FR
     count,
 };