feat(notifications): window border on the balloons

The balloons now wear the Hyprland window border: 2px, the udt gradient
read from ~/.cache/udt/udt-border.lua (the file look_and_feel.lua reads,
watched so a wallpaper change follows), rotating, with the windows'
corners.

A Rectangle border takes one colour and its radius is circular, while the
windows use rounding 15 at rounding_power 4, a superellipse. So the body
and the border are both drawn by a Shape from an SVG path whose corners
are the cubic fit to |x|^4 + |y|^4 = 1, and the border is a ring filled
odd-even, which keeps the translucent body from showing the gradient
through.

One angle drives every balloon, so they turn together, and the
animation runs only while the stack is mapped. Hyprland's borderangle
(speed 30) reads as 3s a turn, which looked slow on a balloon that size,
so the turn is 1.5s, tuned by eye.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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