Dark mode 🙃

[Chuloo]

User request https://x.com/arefbuilds/status/2099975730180575627?s=20

[agryash]

Can I work on this @Chuloo ?

[wuqqers]

PR #130 adds dark mode support to the Android app. Here's what it covers:

• A new MuralColorSet data class holds both light and dark palettes. The theme swaps the active set at composition time, so all existing color references follow automatically without touching individual screens.
• Settings gets an "Appearance" section with a toggle to switch between light and dark.
• The dark palette uses warm brown undertones (#1A1614, #1E1B16) instead of pure grey. Accent colors shift a notch brighter to hold contrast on dark backgrounds.
• Hardcoded Color.White references in the nav bar, text fields, and action buttons were replaced with theme-aware values, so everything adapts properly.

Tested on Android emulator. No iOS changes yet.