Improve panel mode exit cleanup

When exiting panel mode, now properly resets:
- Remove zoom attribute to clear any zoom state
- Scroll to top position to reset viewport

This ensures a clean state when transitioning back to regular reading mode.

fixed-layout.js

@@ -529,6 +529,8 @@ export class FixedLayout extends HTMLElement {
     this.#currentPanels = [];
     this.#hidePanelOverlay();
     this.removeAttribute("panel-mode");
+    this.removeAttribute("zoom");
+    this.scrollTo(0, 0);
     this.#render();
   }