refactor: solid-color palette refactor (BrandSky, removal of gradients)

- Replace BrandPink/BrandIndigo with BrandSky variants (light/dark).
- Add BrandRose600, BrandAmber600, BrandEmerald600.
- Add BackgroundCream for light mode background.
- Switch light theme background from white to BackgroundCream.
- Keep wordmark gradient (only intentional gradient in brand).

Upstream: a60ea08

android/app/src/main/java/com/miti99/loto/ui/theme/Theme.kt

@@ -13,7 +13,7 @@ private val LotoLightColorScheme = lightColorScheme(
     onSecondary = BackgroundLight,
     tertiary = BrandEmeraldLight,
     onTertiary = BackgroundLight,
-    background = BackgroundLight,
+    background = BackgroundCream,
     onBackground = Color0F172A,
     surface = SurfaceLight,
     onSurface = Color0F172A,