Add live vectorization dialog for image layers

Vectorize now opens a dialog with a live preview instead of applying a fixed trace. Presets cover poster, photo and black-and-white, and you can tune colours, speckle filtering and curve fitting the same way vtracer’s own app does. Tracing runs in the browser, so the preview matches the result and nothing is written to the document until you apply it. The traced colours land as one layer each, grouped above the image, and undo takes the whole conversion back in one step.