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_flutter/test/app/train_app_test.dart

@@ -209,7 +209,7 @@ void main() {
           ),
           appVersion: (version: '0.1.0', build: 7),
           sourceCodeUrl: () async =>
-              'https://code.halkyon.app/project-halkyon/halkyon',
+              'https://code.halkyon.app/halkyon-days/halkyon',
           openPerson: (person) async {
             opened.add(person);
             return TrainPerson(
@@ -721,7 +721,7 @@ void main() {
 
     expect(find.text('The source of what this instance runs:'), findsOneWidget);
     expect(
-      find.text('https://code.halkyon.app/project-halkyon/halkyon'),
+      find.text('https://code.halkyon.app/halkyon-days/halkyon'),
       findsOneWidget,
     );
   });