Add volume dashboard widget and extract shared volume helpers

[multiplex55]

Motivation

• Share system/process volume enumeration and helpers so both the volume dialog and dashboard widget can reuse the same code.
• Provide a dashboard widget to control system volume and per-process audio sessions with mute toggles and periodic/manual refresh.
• Cache volume data and make refresh behavior configurable to avoid frequent expensive enumeration.

Description

• Extracted ProcessVolume, get_system_volume and get_process_volumes into a reusable module at src/gui/volume_data.rs and exported it via pub(crate) mod volume_data in src/gui/mod.rs.
• Updated src/gui/volume_dialog.rs to use the new volume_data helpers instead of duplicating enumeration/mute logic.
• Added a new dashboard widget src/dashboard/widgets/volume.rs that exposes a VolumeConfig, caches a VolumeSnapshot, shows a system volume slider + mute button, lists audio sessions with sliders and mute buttons, and emits existing launcher actions like volume:set:<n>, volume:mute_active, volume:pid:<pid>:<level>, and volume:pid_toggle_mute:<pid>.
• Registered the widget in the default registry in WidgetRegistry::with_defaults (src/dashboard/widgets/mod.rs).

Testing

• No automated test suite was executed as part of this change.
• A unit test slider_change_unmutes was added to src/gui/volume_data.rs to verify ProcessVolume::slider_changed behavior but it was not run here.
• Project build and integration testing were not performed in this rollout.
• Manual verification steps (UI run) were not executed in the CI environment.

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Codex Task