feat(displays): add night light settings UI in displays

[mateoalfaro]

What

Adds a Night Light group to the Displays page, complementing the existing quick tile, which only toggles Night Light and already reflects the schedule-aware effective state.

• Adds a master SwitchRow bound to night-light-enabled via settings.bind(..., switch_btn, ...), following the existing codebase pattern.

• Adds an Adaptive Schedule switch that shows or hides the schedule controls.

• Adds From and To TimePicker rows that write through the manager setters.

• Adds a temperature Gtk.Scale covering 1000–6500 K in 100 K steps.

• Keeps controls synchronized with external changes, including:

  • dconf changes
  • quick-tile changes
  • schedule transitions

  Synchronization is handled through NightLightManager.changed, with SignalHandler.block_matched preventing feedback loops.

• Updates the master row subtitle to reflect the effective state:

  • On
  • Off
  • Off until 19:00 when adaptive scheduling is enabled

No new dependencies are introduced. The implementation uses the existing PreferencesGroup, SwitchRow, and TimePicker widgets.

Dependencies

Depends on the corresponding prs:

• libsingularity changes for the manager API
• singularity-desktop changes for the settings schema