Adds a table-driven dialog over RenderStyle.STYLE_FIELDS (colour swatches,
an opacity spin and a label font size), reachable from Options. Edits
preview immediately on every canvas, Cancel restores the style the dialog
opened with, and only OK persists — as JSON under QSettings plot/props,
reloaded on the next launch. Both canvases gain set_style(); the dialog
deliberately exposes plot_style() rather than style() so QWidget.style()
keeps its Qt meaning.
