feat(monthly): add support for MONTHLY recurrence (P07, P08)

- Add `monthly_pattern`, `simple_monthly` types and `monthly` field to
  `rem`
- Implement `render_monthly` and `add_until_monthly` in `remind.ml`
- Handle `BYMONTHDAY` (P07) and nth-weekday `BYDAY` (P08) patterns in
  `eventPredicates.ml`
- Add `add_months` utility for date arithmetic
- Mark P07 and P08 as implemented in documentation

bin/utils.ml

@@ -160,6 +160,20 @@ let get_rdates ev =
     Printf.eprintf "Found RDATE with periods: %d entries; UID: %s\n" (List.length periods) uid;
   []
 
+let add_months (date : Timedesc.Date.t) (n : int) : Timedesc.Date.t =
+  let year = Timedesc.Date.year date in
+  let month = Timedesc.Date.month date in
+  let day = Timedesc.Date.day date in
+  let total_months = (year * 12) + (month - 1) + n in
+  let new_year = total_months / 12 in
+  let new_month = (total_months mod 12) + 1 in
+  let rec try_day d =
+    match Timedesc.Date.Ymd.make ~year:new_year ~month:new_month ~day:d with
+    | Ok date -> date
+    | Error _ -> try_day (d - 1)
+  in
+  try_day day
+
 let get_recurrence_id ev =
   List.find_map
     (fun prop ->