Shell/UI: Expose PanelSlider corner radii as properties

[JaxonWright]

Ui/PanelSlider.qml hardcodes the corner radius of the three parts it draws: the track (height / 2), the tick marks (1), and the knob (knobSize / 2). Everything else about the component is already themeable through properties.

That leaves one option for a plugin that wants square controls. Copy the file. I hit this in a third-party WaveBar media widget, where the vendored copy now has to be diffed against this one on every release to catch changes, and that diff is three lines.

This lifts the three values into properties whose defaults are the expressions they replace:

property real trackRadius: trackHeight / 2
property real tickRadius: 1
property real knobRadius: knobSize / 2

The track Rectangle's height is root.trackHeight, so trackHeight / 2 is the value height / 2 already produced. The fill binds radius: track.radius, so it follows the track and needs no property of its own.

Existing callers set none of these. plugins/panels/audio/Panel.qml, plugins/panels/monitor/Panel.qml, and the dev gallery render exactly as before.

qmllint -I shell reports the same warnings as it does on the current file, plus one on radius: root.tickRadius. That is the unqualified-access class the surrounding lines in the same Repeater delegate already produce, color: root.tickColor among them.

🤖 Generated with Claude Code

[JaxonWright]

Added test/shell.d/panel-slider-radius-test.sh, following the shape of button-border-stability-test.sh. It pairs a source assertion with a headless Quickshell fixture behind the usual require_compositor and quickshell guards.

It covers both halves of the change. The defaults still resolve to trackHeight / 2, 1 and knobSize / 2, and every corner the component draws stays rounded. Setting the three properties to 0 squares all of them, which is the case that previously required copying the file.

The fixture walks the slider's children collecting anything with a radius, rather than reaching for the track, fill and knob by declaration order, so it survives the component gaining a drawn part.

I checked it against the pre-change component, where it fails on Cannot assign to non-existent property "knobRadius". ./test/shell is otherwise unchanged. 4 of 237 files fail here, and the same 4 fail on an unmodified quattro checkout, because they want an omarchy-pkgs checkout that my machine doesn't have.