chore(release): prepare v2.8.0

Bump versionName to 2.8.0 (versionCode 20800) — the merge of this to main
is what cuts the release. Move the accumulated 2.8.0 work out of
[Unreleased] into a dated CHANGELOG section and regenerate the F-Droid
per-version changelog.

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 = 20705
-        versionName = "2.7.5"
+        versionCode = 20800
+        versionName = "2.8.0"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }