feat: add --timezone CLI option for configurable output timezone

bin/commandLine.ml

@@ -5,12 +5,16 @@ let files =
   let doc = "Files to process" in
   Arg.(non_empty & pos_all string [] & info [] ~docv:"FILE" ~doc)
 
+let timezone =
+  let doc = "Target timezone for output (e.g. Europe/Rome). Defaults to local timezone." in
+  Arg.(value & opt (some string) None & info [ "timezone"; "z" ] ~docv:"TZ" ~doc)
+
 let main_command f =
   let doc = "Convert iCalendar files to remind format" in
   let man = [] in
   Cmd.make (Cmd.info "ical2rem" ~version:"%%VERSION%%" ~doc ~man)
   @@
-  let+ files = files in
-  f files
+  let+ files = files and+ tz = timezone in
+  f tz files
 
 let main f = Cmd.eval @@ main_command f