SharedCanvas: Refactor ResizeObserver Architecture & Event Routing

Context

During work on the Header Canvas (#9313), we discovered a fundamental architectural flaw in how Neo.app.SharedCanvas handles ResizeObserver events. The symptom was that onResize() never fired when the browser window resized, breaking the synchronization between the DOM and the Shared Worker canvas renderer.

The Flaw

1. src/component/Canvas.mjs uses monitorSize_: true to automatically add a resize DOM listener to me.getCanvasId(). When it resizes, it calls onDomResize(data), which simply fires a Neo component event: this.fire('resize', data).
2. src/app/SharedCanvas.mjs (which extends component/Canvas) maps this Neo component event via listeners: { resize: 'onResize' }.

This is structurally flawed: Listening to a component event that your own base class fires is an anti-pattern. Events are intended for communication between unrelated instances (e.g., a Viewport listening to a Button's click). A subclass should simply override the method (onDomResize) and call super.onDomResize(), rather than registering an event listener against itself.

The Second Breaking Point: If the canvas is absolutely positioned (like the .app-header-canvas with width: 100%), the browser's native ResizeObserver often does not trigger on the canvas itself when the parent container (the Toolbar) resizes. Because the element being observed (me.id) doesn't trigger, manager.DomEvent never routes a resize event back to the component, and the whole chain is dead.

The Patch (Current State)

We temporarily patched this in src/app/header/Canvas.mjs by adding a getObserverId() method that returns this.parentId instead of this.id. However, this still requires manually adding parent.addDomListeners({ resize: me.onToolbarResize }), bypassing the SharedCanvas default setup entirely.

The Goal

This ticket is to refactor the base SharedCanvas and component/Canvas to have a unified, clean architecture for resize tracking:

• Remove the listeners: { resize: 'onResize' } anti-pattern from SharedCanvas. It should instead override onDomResize.
• Allow configuration of which node triggers the component's size monitoring (e.g., observing a parent flex container when the canvas is absolutely positioned), so the base class monitorSize logic works reliably without manual addDomListeners workarounds in subclasses.