run optional command after switching audio sink/source

[romanstingler]

Closes #892

Allows an optional command to run automatically after switching the audio output or input device in the settings panel. Useful for working around PipeWire/WirePlumber routing issues (e.g. RAOP/HomePod sinks) that require restarting the audio service on switch.

• Add audio_sink_post_switch_cmd and audio_source_post_switch_cmd to SettingsModuleConfig / AudioSettingsConfig.
• Track a pending_* flag when ashell issues a default sink/source switch, then run the command once the ServerInfo event confirms the default actually changed. Re-selecting the current default is treated as a no-op and does not trigger the command.
• Commands run asynchronously (fire-and-forget, bash -c) via crate::utils::launcher::execute_command.
• Update docs (full_config.md, settings.md).

Example config

[settings]
audio_sink_post_switch_cmd = "systemctl --user restart wireplumber"