pagescene: add applyJson + Reset-to-original menu item

Adds Editor.applyJson(jsonString) that delegates to the existing
save_manager.loadCache JSON-deserialize path and additionally clears
the undo stack and emits SceneResetEvent. Used by the new Reset flow
so the user can't "undo" a reset back into their discarded edits.

PageSceneAdapter gains an optional resetToOriginal() hook. When the
host implements it AND a sceneToken is loaded, the File menu shows a
destructive "Reset to original" item that opens a confirm modal and
then defers to the host (which knows where to find the original
snapshot — the webapp's per-token cache lands in a follow-up commit).
Tauri leaves the hook off so nothing changes there.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/libs/components/pagescene/src/lib/adapter.ts

@@ -209,6 +209,15 @@ export interface PageSceneAdapter {
   // is always signed in. Webapp host wires this to its auth modal.
   promptSignup?(reason?: string): void;
 
+  // Roll the current editor session back to the original scene the
+  // host loaded. Called when the user confirms the destructive Reset
+  // modal in the File menu. Implementations typically pull the
+  // original scene JSON from a host-side cache and feed it back
+  // through `editor.applyJson(json)`. When undefined the Reset menu
+  // item is hidden — Tauri leaves it off; webapp wires it to its
+  // per-token cache's stored `original` snapshot.
+  resetToOriginal?(): Promise<void> | void;
+
   // Optional event hooks — telemetry, host-side modals, tab title sync.
   onSelectionChange?(
     sel: { uuid: string; assetType: string } | null,

frontend/libs/components/pagescene/src/lib/comps/ControlsTopButtons/ControlsTopButtons.tsx

@@ -284,6 +284,41 @@ export const ControlsTopButtons = () => {
                     dialogProps: saveAsCopyDialogProps,
                   },
                 ]),
+            // Reset-to-original — destructive, only meaningful when a
+            // sceneToken is in play (URL-loaded scene) AND the host
+            // implements the reset adapter (the webapp does via its
+            // per-token cache; Tauri leaves it undefined and the item
+            // simply doesn't render).
+            ...(sceneMeta.token && editor?.adapter.resetToOriginal
+              ? [
+                  {
+                    label: "Reset to original",
+                    className:
+                      "text-error-400 hover:bg-error-500/15 focus:bg-error-500/15",
+                    divider: true,
+                    dialogProps: {
+                      title: "Reset scene to original?",
+                      content: (
+                        <h4>
+                          This will discard every change you&apos;ve made
+                          in this session and restore the scene to the
+                          version the author shared. This cannot be
+                          undone.
+                        </h4>
+                      ),
+                      confirmButtonProps: {
+                        label: "Reset",
+                        variant: "destructive" as const,
+                        onClick: async () => {
+                          await editor.adapter.resetToOriginal?.();
+                        },
+                      },
+                      closeButtonProps: { label: "Cancel" },
+                      showClose: true,
+                    },
+                  },
+                ]
+              : []),
           ]}
         />
 

frontend/libs/components/pagescene/src/lib/engine/editor.ts

@@ -770,6 +770,18 @@ class Editor {
     this.selection.refreshOutliner();
   }
 
+  // Replace the active scene with the provided serialized JSON, clear
+  // the undo stack, and fire SceneResetEvent. Used by the Reset-to-
+  // original flow so the user can't "undo" the reset back into their
+  // edits. Delegates to save_manager.loadCache which already owns the
+  // JSON-string deserialization path used on initial mount.
+  public async applyJson(jsonString: string) {
+    await this.save_manager.loadCache(jsonString);
+    this.history.clear();
+    this.bus.emit(new SceneResetEvent());
+    this.selection.refreshOutliner();
+  }
+
   public async saveScene({
     sceneTitle,
     sceneToken,