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/_entity-selector.scss

@@ -261,7 +261,7 @@
             color: $es-primary;
         }
 
-        .material-icons {
+        > i {
             font-size: 20px !important;
         }
     }