MiniABS 1.5: Audiobookshelf icon and purple theme

- Launcher icon from the Audiobookshelf icon pack (art/icon_master_512.png),
  including an xhdpi size
- Notification icon cut out of the same artwork; player icons redrawn in the
  icon's lilac (tools/make_icons.py)
- Purple colour scheme taken from the icon: gradient accent buttons, lilac
  text accents (accent_light), purple-tinted backgrounds
- README: where the icon and colours come from

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

res/drawable/btn_flat.xml

@@ -4,7 +4,7 @@
         <shape>
             <solid android:color="@color/pressed" />
             <corners android:radius="6dp" />
-            <stroke android:width="1dp" android:color="@color/accent" />
+            <stroke android:width="1dp" android:color="@color/accent_light" />
         </shape>
     </item>
     <item android:state_enabled="false">