build: the organisation on code.halkyon.app is halkyon-days

ADR 0045, D12, step 1: project-halkyon was renamed halkyon-days, its
repositories keeping their names. Every URL naming it moves along: the
servers' source-code links, which the AGPL requires, and their tests;
the forks in the pubspecs and the lock (appflowy-editor, pure-zstd,
serverpod-offline-sync); the snaps, the F-Droid listings and the
release configurations; the docs. The curator's credit for the
alphabets names the repository by its name since ADR 0015, halkyon,
rather than halkyon_learn. Forgejo redirects the old addresses until
the name is taken again.

ADRs 0015 and 0039 keep the name they were written with, and the local
folders theirs.

halkyon_editor's gen-l10n caches under build/, committed before its
.gitignore named the folder, are no longer tracked.

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

apps/train/halkyon_train_server/lib/src/train_base.dart

@@ -5,8 +5,7 @@ import 'package:halkyon_train_server/src/train_data.dart';
 /// unless its operator sets `SOURCE_URL`. Both licences require offering it
 /// to the people who use the instance (AGPL-3.0-or-later for the app,
 /// SSPL-1.0 for the server).
-const upstreamSourceCodeUrl =
-    'https://code.halkyon.app/project-halkyon/halkyon';
+const upstreamSourceCodeUrl = 'https://code.halkyon.app/halkyon-days/halkyon';
 
 /// The oldest build of the app this server works with (the number after `+`
 /// in the app's version); apps below it are asked to update. Raise it in the