feat(ai): remove OpenPencil built-in icon picker collection

All icons in icon-resolver.ts now carry iconId (e.g. feather:search),
so after AI generation each node automatically maps to the correct
library in the property panel picker. The built-in "OpenPencil"
collection in the picker is no longer needed.

Removes BUILTIN_ICONS import, BUILTIN_ICON_MAP, buildBuiltinSvg,
getIconColorRaw, and all openpencil-specific branches from the dialog.

src/components/shared/icon-picker-dialog.tsx

@@ -68,6 +68,7 @@ export default function IconPickerDialog({
   // Load all icons for the active collection
   useEffect(() => {
     if (!activeCollection) return
+
     setCollectionLoading(true)
     setAllIcons([])
     setTotalCount(null)
@@ -278,7 +279,7 @@ export default function IconPickerDialog({
               return (
                 <button
                   key={iconId}
-                  title={name}
+                  title={iconId}
                   onClick={() => handleSelect(iconId)}
                   disabled={isFetching}
                   className={`w-10 h-10 flex items-center justify-center rounded transition-colors disabled:opacity-50 ${