Let SVG of ui.pyplot fill its container

[falkoschindler]

This PR resolves discussion #3277 by setting the width and height of an SVG element to 100% of its parent container, which is the ui.pyplot element itself. This way the following plot stays inside the ui.card, which wasn't the case before:

with ui.card().classes('w-[400px]'):
    with ui.pyplot() as plot:
        plot.fig.gca().plot()