Inhibit idle/sleep during audio/video playback (flakes-va02)

Add sway-audio-idle-inhibit to prevent hypridle from powering off monitors when audio is playing (e.g. YouTube, Netflix in Chrome). Changes: add package to hypridle.nix and spawn at niri startup.

Summary of Changes

• Added sway-audio-idle-inhibit package to mixins/home/gui/linux/services/hypridle.nix
• Added spawn-at-startup "sway-audio-idle-inhibit" to niri config (config.common.kdl)

The daemon monitors PipeWire/PulseAudio for active audio streams and sends a Wayland idle inhibit signal, preventing hypridle from powering off monitors during video/audio playback.