[FIX] web: _getFullCalendarOptions firstDay key

After making improvement to allow user to choose first day of week per
language, it was not taken into account that calendar view uses
different weekdays indexing than newly added field `week_start`. The
difference is in Sunday, where in calendar view it has index 0, but
week_start index is 7 (other days' indexes match).

opw 1916666

closes odoo/odoo#29471

addons/web/static/src/js/views/calendar/calendar_model.js

@@ -385,6 +385,9 @@ return AbstractModel.extend({
      * @returns {Object}
      */
     _getFullCalendarOptions: function () {
+        var week_start = _t.database.parameters.week_start || 0;
+        // calendar uses index 0 for Sunday but Odoo stores it as 7
+        week_start = week_start % 7;
         return {
             defaultView: (this.mode === "month")? "month" : ((this.mode === "week")? "agendaWeek" : ((this.mode === "day")? "agendaDay" : "agendaWeek")),
             header: false,
@@ -404,7 +407,7 @@ return AbstractModel.extend({
             monthNamesShort: moment.monthsShort(),
             dayNames: moment.weekdays(),
             dayNamesShort: moment.weekdaysShort(),
-            firstDay: _t.database.parameters.week_start,
+            firstDay: week_start,
             slotLabelFormat: _t.database.parameters.time_format.search("%H") != -1 ? 'H:mm': 'h(:mm)a',
         };
     },