previewPanelWidth top-level constant

The rectangle an entry under no canvas is framed as, in logical pixels at a ratio of 1.

Both backends have to agree about it, which is why it is one constant and not two. The embedder guest reads it as CaptureViewport.panel and the tester harness stages it when canvasFor finds nothing; a harness that simply left the test surface alone would judge the same entry on flutter_test's 800×600 instead — measured, and it invented two overflows the guest did not report.