feat: the F-Droid repository moves to halkyon.app

The repository is Halkyon's, for all of its apps, so its address is the
brand's: https://halkyon.app/fdroid/repo. learn.halkyon.app stays Halkyon
Learn's own address, for its website later. Moved before anyone but the
maintainer had added it; the address is permanent once people have.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

tool/fdroid_repo.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Updates the F-Droid repository at https://learn.halkyon.app/fdroid/repo and
+# Updates the F-Droid repository at https://halkyon.app/fdroid/repo and
 # uploads it: adds an APK if one is given, signs the index with the repository's
 # key, and uploads everything to ramiel as the user fdroid, whose key can only
 # write into the folder Caddy serves (rrsync). tool/release.sh runs it with each
@@ -26,7 +26,7 @@ keystore="${HALKYON_FDROID_KEYSTORE:-$HOME/.local/share/halkyon-learn/fdroid-ind
 readonly key_alias='halkyon-fdroid'
 readonly pinned_file='tool/release/fdroid-index-certificate.sha256'
 readonly upload_host='ramiel-fdroid'
-readonly repo_url='https://learn.halkyon.app/fdroid/repo'
+readonly repo_url='https://halkyon.app/fdroid/repo'
 
 fail() {
   echo "✗ $*" >&2