release: bump to 3.0.0 and refresh F-Droid metadata

Bump versionCode to 13 and versionName to 3.0.0. The Flutter app ended at
2.0.0-alpha.3+12 (versionCode 12), so 13 keeps the eventual com.subtracks
release an in-place upgrade.

Metadata was still inherited from the Flutter app:
- add the changelog for versionCode 13
- rewrite the descriptions for the native app
- replace the Flutter phone screenshots with native renders, and mute the
  screenshot fixture palette so the placeholder artwork reads as covers

app/src/test/kotlin/com/subtracks/ui/AppScreenshotTest.kt

@@ -741,7 +741,7 @@ private object Fixtures {
 
         fun color(offset: Float): Int =
             Color.HSVToColor(
-                floatArrayOf(((base + offset) % 360f + 360f) % 360f, 0.88f, 0.92f),
+                floatArrayOf(((base + offset) % 360f + 360f) % 360f, 0.42f, 0.72f),
             )
 
         val palette =