Add system controls dashboard widget (brightness, volume, power plans)

[multiplex55]

Motivation

• Provide a compact dashboard widget offering common system controls (brightness slider, volume slider + mute toggle, and power plan selector) and surface current system state.
• Reuse existing action/launcher hooks where available and add minimal helpers where needed to query and set system state.

Description

• Add a new widget SystemControlsWidget at src/dashboard/widgets/system_controls.rs that uses a TimedCache to refresh and render brightness, volume/mute, and power-plan controls and shows per-control fallback/error messages when features are unavailable.
• Expose system helpers in src/actions/system.rs: get_system_volume, get_system_mute, get_main_display_brightness, get_power_plans, set_power_plan and a PowerPlan struct, plus a Windows-only parser parse_powercfg_list for powercfg /L output.
• Wire new actions into the launcher and action parsing in src/launcher.rs: add ActionKind::VolumeToggleMute and ActionKind::PowerPlanSet, parse volume:toggle_mute and power:plan:set:<guid>, call system::toggle_system_mute() and system::set_power_plan() in the dispatch path, and add a toggle_system_mute helper in the launcher implementation.
• Register the widget in the dashboard registry by updating src/dashboard/widgets/mod.rs and implement UI behavior to dispatch the existing action strings (e.g. brightness:set:<n>, volume:set:<n>, volume:toggle_mute, power:plan:set:<guid>).

Testing

• Added unit tests covering action parsing and powercfg output parsing (src/launcher.rs tests for parsing new action prefixes and src/actions/system.rs Windows-only test for parse_powercfg_list).
• Ran cargo test --lib in this environment but the build failed before executing tests due to a missing system library required by alsa-sys (pkg-config/alsa.pc not present), so the new unit tests were not executed here.
• Local/unit test intent: cargo test --lib (would run added parse tests); execution blocked by environment dependency failure as noted above.

---

Codex Task