[AF-402] Fix model reinitialization issues with viewport and internal…

[piotrblaszczyk]

Problem

Two issues occurred when reinitializing the same model:

1. Viewport issues: After model reinitialization, zoomToFit didn't work and linking operations moved the whole viewport. Root cause: viewport.width and viewport.height were undefined because ResizeObserver doesn't fire when element size hasn't changed.

2. Angular tracking issues: When the same model with identical node IDs was reinitialized, Angular's @for loop with trackBy using id couldn't distinguish between old and new nodes. The _internalId (used for forcing view recreation) was only assigned by middleware when nodes were added via transactions, not during initial model creation.

Solution

1. Viewport fix: Added getViewportSize() callback to FlowCore and call initializeViewportSize() before render() to ensure viewport dimensions are set from element's getBoundingClientRect() on every initialization.

2. Internal ID fix: _internalId is now assigned during initializeModel() as well, not only when adding nodes via middleware.