fix: eliminate double scrollbar in script editor and extend page scroll area

ScriptBodyEditor: overlay now has overflow-hidden and syncs scroll
position from the textarea via onScroll, so only one scrollbar appears.
ScriptManagePage: outer scroll container is now full-width so scrolling
works even when hovering outside the max-w content area.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/ScriptManagePage.tsx

@@ -27,16 +27,18 @@ export default function ScriptManagePage() {
   }
 
   return (
-    <div className="p-6 max-w-5xl mx-auto h-full overflow-y-auto">
-      {mode === 'list' ? (
-        <ScriptTemplateListView onEdit={handleEdit} onCreate={handleCreate} />
-      ) : (
-        <ScriptTemplateEditor
-          templateId={editingId}
-          onBack={handleBack}
-          onSaved={handleSaved}
-        />
-      )}
+    <div className="h-full overflow-y-auto">
+      <div className="p-6 max-w-5xl mx-auto">
+        {mode === 'list' ? (
+          <ScriptTemplateListView onEdit={handleEdit} onCreate={handleCreate} />
+        ) : (
+          <ScriptTemplateEditor
+            templateId={editingId}
+            onBack={handleBack}
+            onSaved={handleSaved}
+          />
+        )}
+      </div>
     </div>
   )
 }