feat(desktop): fixture color presets

[johncarmack1984]

Summary

• Replace the single Reading Light button in the fixture color popover with a 2-column grid of 8 per-fixture presets
• All presets at 40% dimmer: Reading Light, Daylight, Fire, Rose, Lavender, Cobalt, Steel, Dusk
• Each preset flows through mixToEmitters so RGBAW fixtures use their Amber and White emitters properly
• Same toggle/restore behavior as before via the preset engine (per-fixture lanes, snapshot/restore)

Test plan

• Open a fixture's color popover — 8 preset buttons visible in a 2×4 grid with colored dot swatches
• Toggle each preset on an RGBAW fixture — verify correct emitter decomposition
• Toggle a preset off — verify prior channel values restore
• Engage one preset over another on the same fixture — verify clean switch
• Engage presets on two different fixtures independently
• Verify presets auto-deactivate when a fader is moved