Modernize Select2 integration and fix popover page jumping

- Upgraded Select2 from bundled v3.4.4 (2013) to v4.x peer dependency
- Removed legacy Select2 v3.x bundled files from lib directory
- Updated Select2 input to use v4.x API (events, AJAX, templates)
- Fixed Bootstrap 5 popover page jumping issue with multi-timeout scroll restoration
- Added comprehensive migration documentation in README-select2-upgrade.md
- Rebuilt all distribution files with updated source code

Breaking Changes:
- Select2 now requires separate installation as peer dependency
- AJAX configuration updated for v4.x format
- Event names changed (select2:select vs select2-loaded)
- Template functions renamed (templateResult vs formatResult)

src/inputs/date/date.js

@@ -26,12 +26,6 @@ $(function(){
 (function ($) {
     "use strict";
     
-    //store bootstrap-datepicker as bdateicker to exclude conflict with jQuery UI one
-    $.fn.bdatepicker = $.fn.datepicker.noConflict();
-    if(!$.fn.datepicker) { //if there were no other datepickers, keep also original name
-        $.fn.datepicker = $.fn.bdatepicker;    
-    }    
-    
     var Date = function (options) {
         this.init('date', options, Date.defaults);
         this.initPicker(options, Date.defaults);
@@ -41,6 +35,18 @@ $(function(){
     
     $.extend(Date.prototype, {
         initPicker: function(options, defaults) {
+            // Initialize bootstrap-datepicker reference
+            if (!$.fn.bdatepicker) {
+                if ($.fn.datepicker) {
+                    $.fn.bdatepicker = $.fn.datepicker.noConflict();
+                    if(!$.fn.datepicker) { //if there were no other datepickers, keep also original name
+                        $.fn.datepicker = $.fn.bdatepicker;    
+                    }
+                } else {
+                    throw new Error('bootstrap-datepicker not found. Please include bootstrap-datepicker before x-editable.');
+                }
+            }
+
             //'format' is set directly from settings or data-* attributes
 
             //by default viewformat equals to format