pipewire: write route volumes without volume step

[faithleysath]

Summary

Fix PipeWire route volume writes for devices that do not report SPA_PROP_volumeStep.

On these devices, volumeStep is parsed as -1. The previous logic only called PwDevice::setVolumes(...) from inside the volumeStep != -1 branch, so route-backed sink volume writes updated Quickshell's local mVolumes state but did not write the new volume to PipeWire.

This keeps the existing minimum-step filtering when a device reports volumeStep, and treats a missing step value as "no step threshold", allowing the route volume write to proceed.

Fixes #807

Testing

• just configure debug -DNO_PCH=ON -DBUILD_TESTING=ON
• just build
• git diff --check
• Tested locally against a Bluetooth sink (bluez_output.6C_5A_B5_EB_5E_8D.1) by writing Pipewire.defaultAudioSink.audio.volumes = [0.24, 0.24] from QML and verifying wpctl get-volume @DEFAULT_AUDIO_SINK@ changed to Volume: 0.24.

just lint-changed was not run because this local checkout does not have the required TIDYFOX environment variable configured.