Fix more Scribe mobile issues

- Add getSavedSelection to check saved selection without consuming it
- Distinguish between setText (replace all) and insertText (at cursor) operations
- Fix replace callback to use setText when no selection exists
- Clean up saved selection state after restoration in HTML utils
- Move mobile editor focus and selection restore to appropriate callbacks

core/lib/utils/html/html_utils.dart

@@ -235,6 +235,7 @@ class HtmlUtils {
           if (selection) {
             selection.removeAllRanges();
             selection.addRange(window._savedRange);
+            delete window._savedRange;
             return selection.toString();
           }
         }
@@ -242,6 +243,17 @@ class HtmlUtils {
       })();''',
     name: 'restoreSelection');
 
+  static const getSavedSelection = (
+    script: '''
+      (() => {
+        if(window._savedRange) {
+          return window._savedRange.toString();
+        } else {
+          return "";
+        }
+      })();''',
+    name: 'getSavedSelection');
+
   static const clearSavedSelection = (
     script: '''
       (() => {