release: cut 2.15.0

Move the Unreleased changelog under [2.15.0] — 2026-07-15, bump
versionName/versionCode to 2.15.0/21500, and sync the F-Droid
per-version changelog.

Milestone 2.15.0 (all integrated): #21 #35 #36 #39 #40 #46 #52.

Co-Authored-By: Claude Opus 4.8 <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 = 21401
-        versionName = "2.14.1"
+        versionCode = 21500
+        versionName = "2.15.0"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }