v6.8.15: the Android app is out for testing

- Android APK download, served from securebit.chat (/downloads/, never
  committed): a "Download Android APK" badge next to the Snap Store one, drawn
  for both themes, and an Android entry in the download menu (recommended on
  Android devices). nginx serves .apk as an attachment; the service worker
  leaves /downloads/ alone. The APK version is one constant in three places,
  checked by tests/desktop-download-links.test.mjs.
- Roadmap versions follow the real releases: Group Communications v6.0
  shipped, Mobile Edition v6.8 is current, Quantum-Resistant v7.0 is in
  development (2027), then v8.0 and v9.0.
- "Download desktop app" is now "Download the app"; the "You're on Web" pill
  in the download menu is gone.

doc/CRYPTOGRAPHY.md

@@ -7,7 +7,7 @@ this document describes.
 
 | | |
 | --- | --- |
-| Release | v6.8.14 |
+| Release | v6.8.15 |
 | Protocol version | 4.1 |
 | Ratchet wire version | 1 |
 
@@ -347,5 +347,5 @@ no leg may stop a capture it borrowed.
 
 ## Scope
 
-This describes the browser implementation as it stands in v6.8.14. It is not a
+This describes the browser implementation as it stands in v6.8.15. It is not a
 substitute for independent cryptographic review.