Bug 42292: (follow-up) Replace removed dateformat_pref var in OPAC calendar.inc

The OPAC port moved the dateformat pref into Koha.addPrefs and dropped
the local `var dateformat_pref = ...;` declaration, but four usages of
`dateformat_pref` remain in the same file (the debug alert, two
`switch` statements, and the get_dateformat_str call in validate_date).
At runtime the switches throw ReferenceError, so flatpickr ends up
with the default 'Y-m-d' format and date validation via validate_date
crashes before showing the invalid-date alert.

Replace all remaining occurrences with Koha.prefs.dateformat, which is
populated by the new addPrefs block at the top of the include.

To test:
- Set dateformat syspref to each of us / metric / dmydot / iso.
- Open any OPAC page using a flatpickr input (e.g. hold date, patron
  dob on the registration form).
- Confirm the widget renders dates in the expected format and
  entering an invalid string triggers the validate_date alert.
- No ReferenceError in the browser console on page load.

Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Jan Kissig <bibliothek@th-wildau.de>
Signed-off-by: Pedro Amorim <pedro.amorim@openfifth.co.uk>

koha-tmpl/opac-tmpl/bootstrap/en/includes/calendar.inc

@@ -21,7 +21,7 @@
     };
     var sentmsg = 0;
     if (debug > 1) {
-        alert("dateformat: " + dateformat_pref + "\ndebug is on (level " + debug + ")");
+        alert("dateformat: " + Koha.prefs.dateformat + "\ndebug is on (level " + debug + ")");
     }
     var calendarFirstDayOfWeek = Koha.prefs.CalendarFirstDayOfWeek;
     var flatpickr_timeformat_string = Koha.prefs.TimeFormat == "12hr" ? "G:i K" : "H:i";
@@ -29,7 +29,7 @@
     var flatpickr_dateformat_string = "";
     var delimiter = "";
     var altinput_dateformat = "";
-    switch (dateformat_pref) {
+    switch (Koha.prefs.dateformat) {
         case "us":
             flatpickr_dateformat_string = "m/d/Y";
             altinput_dateformat = "mm/dd/yyyy";