feat: the apps' icons as one family, drawn by one tool (ADR 0031)

tool/app_icons.py at the root replaces Learn's and Todo's scripts: each
app's Lucide glyph (lucide-static 1.50.0) drawn by librsvg, fading from
cobalt through the colours ADR 0031 names, then every platform's
launcher icon through flutter_launcher_icons, Windows' .ico with every
size, and Learn's snap icon.

Learn's icon becomes the graduation cap, its window's on Linux too, with
Lucide's notice in its licence list; the kingfisher stays the emblem on
the rail and the launch screen. Todo's becomes list-todo, Train's bike.

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/learn/halkyon_learn_flutter/test/app/app_icon_license_test.dart

@@ -0,0 +1,25 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/app/app_icon_license.dart';
+
+void main() {
+  testWidgets(
+    "Lucide's notice, whose graduation cap is Learn's icon, joins the "
+    'licence list',
+    (tester) async {
+      registerAppIconLicense();
+
+      final entries = await tester.runAsync(
+        () => LicenseRegistry.licenses.toList(),
+      );
+      final lucide = entries!.where(
+        (entry) => entry.packages.contains('Lucide (the app icon)'),
+      );
+      expect(lucide, isNotEmpty);
+      expect(
+        lucide.first.paragraphs.map((paragraph) => paragraph.text).join('\n'),
+        contains('ISC License'),
+      );
+    },
+  );
+}