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/containers/editable-popover5.js

@@ -3,7 +3,6 @@
  * ---------------------
  * requires bootstrap-popover.js
  */
-import { Popover } from "bootstrap";
 
 (function ($) {
     "use strict";
@@ -13,7 +12,7 @@ import { Popover } from "bootstrap";
         containerName: 'popover',
         containerDataName: 'bs.popover',
         innerCss: '.popover-body',
-        defaults: Popover.Default,
+        defaults: bootstrap.Popover.Default,
 
         initContainer: function(){
             $.extend(this.containerOptions, {
@@ -40,7 +39,27 @@ import { Popover } from "bootstrap";
 
         /* show */
         innerShow: function () {
+            // Preserve scroll position to prevent page jumping
+            var scrollTop = $(window).scrollTop();
+            var scrollLeft = $(window).scrollLeft();
+            
             this.call('show');
+            
+            // Multiple restoration attempts to handle async positioning
+            setTimeout(function() {
+                $(window).scrollTop(scrollTop);
+                $(window).scrollLeft(scrollLeft);
+            }, 0);
+            
+            setTimeout(function() {
+                $(window).scrollTop(scrollTop);
+                $(window).scrollLeft(scrollLeft);
+            }, 10);
+            
+            setTimeout(function() {
+                $(window).scrollTop(scrollTop);
+                $(window).scrollLeft(scrollLeft);
+            }, 50);
         },
 
         /* hide */
@@ -64,7 +83,7 @@ import { Popover } from "bootstrap";
         call: function() {
             if ( ! $(this.$element).data(this.containerDataName)) {
                 $(this.$element).data(this.containerDataName,
-                    Popover.getOrCreateInstance(this.$element, this.containerOptions)
+                    bootstrap.Popover.getOrCreateInstance(this.$element, this.containerOptions)
                 );
             }