Add scroll position and source context to zoom/pan event data in FixedLayout

This commit enhances the FixedLayout component by including additional
contextual information when zoom or pan operations occur:

- scrollLeft and scrollTop: Current scroll position of the viewport
- source: Origin of the zoom/pan action (e.g., mouse wheel, pinch zoom, button)
- leftWidth: Width of the left page/panel in dual-page layouts

These parameters provide downstream handlers with complete state information
needed to implement sophisticated zoom behaviors, such as:
- Preserving scroll position during zoom operations
- Differentiating between user-initiated and programmatic zoom changes
- Handling asymmetric layouts with different panel widths

This change is part of the ongoing zoom control implementation for
fixed-layout e-books.

fixed-layout.js

@@ -288,8 +288,12 @@ export class FixedLayout extends HTMLElement {
       cursorY,
       dragOffsetX: this.dragOffset.x,
       dragOffsetY: this.dragOffset.y,
+      scrollLeft: this.scrollLeft,
+      scrollTop: this.scrollTop,
       newScrollX,
       newScrollY,
+      source,
+      leftWidth,
     });
     // Apply zoom
     this.setAttribute("zoom", newScale);