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/xml/settings.xml

@@ -50,7 +50,7 @@
     <PreferenceCategory android:title="Acerca de">
         <Preference
             android:key="about"
-            android:title="MiniABS 1.4"
+            android:title="MiniABS 1.5"
             android:summary="Un pequeno cliente de podcasts de Audiobookshelf para Android 2.2+" />
     </PreferenceCategory>
 </PreferenceScreen>