fix: use event TZID for UNTIL date conversion

Add a `tz` field to `rem` to carry the event's DTSTART timezone, and
introduce `timedesc_of_utc_or_timestamp_tz` to convert UNTIL timestamps
in that timezone instead of the process locale. This makes UNTIL
comparisons locale-independent for events with a known TZID.

bin/eventPredicates.ml

@@ -202,7 +202,14 @@ let collect_start_end_duration rem ev : (Remind.rem, error) result =
           end)
   | `Datetime datetime -> begin
       let start_td = Utils.timedesc_of_timestamp datetime in
-      let rem = { rem with Remind.date = Timedesc.date start_td; Remind.time = Some (Timedesc.time start_td) } in
+      let rem =
+        {
+          rem with
+          Remind.date = Timedesc.date start_td;
+          Remind.time = Some (Timedesc.time start_td);
+          Remind.tz = Some (Timedesc.tz start_td);
+        }
+      in
 
       match ev.dtend_or_duration with
       | None -> Ok rem