release: cut 2.16.0

Locale-aware calendar titles (#60): Month, Week and Day move onto the shared
formatter Agenda already used, the year drops out while you're in the current
one, and the Week title names its month rather than restating the day numbers
printed directly below it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -28,8 +28,8 @@ android {
         // which builds this version and then creates the matching vX.Y.Z tag +
         // release itself (versionCode is pinned to MAJOR*10000 + MINOR*100 +
         // PATCH from versionName, e.g. 2.7.2 -> 20702). See docs/RELEASING.md.
-        versionCode = 21500
-        versionName = "2.15.0"
+        versionCode = 21600
+        versionName = "2.16.0"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }