feat(app): choose a studyset's licence, its year and holder

The studyset sheet gains Licence, as before: "Help me choose" asks three
questions (keep any rights? commercial use? adaptations, alike?) and
recommends one of the six current Creative Commons licences or CC0, with
what it lets others do and a link to its terms; the licence field
suggests well-known licences and takes any other as typed, kept in one
spelling (`cc by-nc-sa 3.0 us` is `CC-BY-NC-SA-3.0-US`); the year sits at
its end (1000 to next year), the holder below. An older or ported
Creative Commons licence gets a note naming its current version. A new
sub-studyset starts with the licence of the one it is in.

Left out until publishing exists: the warnings about sub-studysets'
licences that a parent's doesn't cover. The licence shows for every
studyset, as there is no visibility yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/test/feature/studyset/model/license_test.dart

@@ -0,0 +1,26 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/feature/studyset/model/license.dart';
+
+void main() {
+  test('a licence is kept in one spelling: known ones as known, Creative '
+      'Commons by its identifier, anything else as typed', () {
+    expect(canonicalLicense('Cc By 4.0'), 'CC-BY-4.0');
+    expect(canonicalLicense('cc_by_sa'), 'CC-BY-SA-4.0');
+    expect(canonicalLicense('public domain'), 'CC0-1.0');
+    expect(canonicalLicense('mit'), 'MIT');
+    expect(canonicalLicense('arr'), 'All Rights Reserved');
+    expect(canonicalLicense('cc by-nc-sa 3.0 us'), 'CC-BY-NC-SA-3.0-US');
+    expect(canonicalLicense('  My   own terms '), 'My own terms');
+    expect(canonicalLicense('  '), isNull);
+    expect(canonicalLicense(null), isNull);
+  });
+
+  test('an older or ported Creative Commons licence has a current version; '
+      'a current one, or another licence, none', () {
+    expect(newerCreativeCommons('CC-BY-NC-SA-3.0-US'), 'CC-BY-NC-SA-4.0');
+    expect(newerCreativeCommons('CC-BY-2.0'), 'CC-BY-4.0');
+    expect(newerCreativeCommons('CC-BY-SA-4.0'), isNull);
+    expect(newerCreativeCommons('CC0-1.0'), isNull);
+    expect(newerCreativeCommons('MIT'), isNull);
+  });
+}