refactor: remove remind_sync wrapper library and inline utilities

The `remind_sync` library was acting as a thin re-export layer. This
commit removes it entirely and moves the only non-trivial utility
(`show_error` for `Timedesc.Date.Ymd.error`) directly into
`bin/utils.ml`. Dead `[@@deriving show]` annotations on `rem`,
`week_first_day`, and `error` types are also removed.

bin/remind.ml

@@ -1,7 +1,6 @@
-open Remind_sync
 open Utils
 
-type week_first_day = [ `Sunday | `Monday ] [@@deriving show]
+type week_first_day = [ `Sunday | `Monday ]
 
 type simple_weekly = {
   count_or_until : Icalendar.count_or_until option;
@@ -9,7 +8,6 @@ type simple_weekly = {
   byday : Icalendar.weekday list;
   week_start : week_first_day option;  (** First day of the week for weekly recurrence *)
 }
-[@@deriving show]
 (** A simple weekly REM command *)
 
 type rem = {
@@ -26,7 +24,6 @@ type rem = {
       *)
   exdate : Icalendar.date_or_datetime list;  (** List of excluded dates for recurring events *)
 }
-[@@deriving show]
 (** A complete REM command *)
 
 let empty =