brand: lift the icon background off black

The near-black #120C12 behind the icon reads as black on dark home
screens, and the same fill backs the store icon, feature graphic and
README logo. Raise it to a plum #2D1C2D (L* 4.6 -> 13.1) so the
background is visible without costing the #F862F8 foreground its
contrast. The amber nightly background moves from #1A1200 to #2E2000 by
the same lightness step, so it stays amber instead of drifting to
brown-black.

Raster assets were recoloured from the committed originals; the vector
sources in images/ have no background and are unchanged.

app/src/main/res/values/ic_launcher_background.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <color name="ic_launcher_background">#120C12</color>
+    <color name="ic_launcher_background">#2D1C2D</color>
 </resources>

app/src/nightly/res/values/ic_launcher_background.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <color name="ic_launcher_background">#1A1200</color>
+    <color name="ic_launcher_background">#2E2000</color>
 </resources>