feat(schema): add night light GSettings keys

[mateoalfaro]

What

Adds five keys to the dev.sinty.desktop schema backing the new settings-driven night light:

Key	Type	Default	Notes
night-light-enabled	b	false	Master switch
night-light-temperature	i	4000	Kelvin, range 1000–6500
night-light-adaptive	b	false	Schedule-based mode
night-light-adaptive-from	s	'19:00'	HH:MM
night-light-adaptive-to	s	'07:00'	HH:MM

Keys follow the existing theme-adaptive* pattern: the same HH:MM schedule model, including support for overnight spans.

The schema change is required before the libsingularity manager update can read these settings. The UI follows in singularity-shell.

Why

Night light is currently a hard-coded on/off toggle at 4000 K, with no persistence, temperature control, or scheduling.