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/utils.ml

@@ -1,8 +1,14 @@
-open Remind_sync
 open Icalendar
 
 type months = Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec
 
+let show_error (e : Timedesc.Date.Ymd.error) : string =
+  match e with
+  | `Does_not_exist -> "Date does not exist"
+  | `Invalid_year y -> Printf.sprintf "Invalid year: %d" y
+  | `Invalid_month m -> Printf.sprintf "Invalid month: %d" m
+  | `Invalid_day d -> Printf.sprintf "Invalid day: %d" d
+
 let string_of_weekday = function
   | `Monday -> "Mon"
   | `Tuesday -> "Tue"