feat: icon abstraction layer for PS 1.6-9.x compatibility

Add esIcon() JS helper and renderIcon() PHP helper that auto-detect
the icon framework (Material Icons on PS 8+/9+, FontAwesome 4 on PS
1.6/1.7) and render appropriate HTML. Includes bidirectional mapping
between FA4 class names and Material Icons names.

- Replace all hardcoded <i class="material-icons"> with esIcon()/renderIcon()
- Add FA4_MAP (Material→FA4) and reverse FA4→Material mapping in both JS and PHP
- Add detectIconMode() with PHP data-attribute hint and font-family probe fallback
- Fix 4 self/this scope bugs in esIcon calls across _methods.js, _tree.js, _preview.js
- Add esIconUpdate() for dynamically updating existing icon elements
- Add normalizeIconName() to handle both FA4 and Material input formats

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sources/scss/components/_chips.scss

@@ -332,7 +332,7 @@
             color: darken($es-primary, 10%);
         }
 
-        i.material-icons {
+        i {
             font-size: 14px;
         }
     }
@@ -831,7 +831,7 @@
             color: $es-text-secondary;
         }
 
-        i.material-icons {
+        i {
             font-size: 18px;
         }
     }
@@ -853,7 +853,7 @@
         color: $es-text-muted;
         font-size: $es-font-size-sm;
 
-        i.material-icons {
+        i {
             font-size: 20px;
         }
 
@@ -868,7 +868,7 @@
         padding: $es-spacing-xl 0;
         color: $es-text-muted;
 
-        i.material-icons {
+        i {
             font-size: 48px;
             opacity: 0.4;
             margin-bottom: $es-spacing-sm;
@@ -978,7 +978,7 @@
         border-bottom: 1px solid $es-border-color;
         background: $es-slate-50;
 
-        i.material-icons {
+        i {
             font-size: 18px;
             color: $es-text-muted;
         }