feat(icons): every icon fades from cobalt into its app's backdrop; Learn shows its own inside the app; Train's dumbbell (ADR 0031, D5 to D7)

Each icon now ends in the colour its app's screens wash down from: Learn cobalt into
purple, Todo through violet into strawberry (Learn's former gradient), Train unchanged
into carrot, Meditate's to come through pacific into fern (Todo's former). Train's glyph
is Lucide's dumbbell, Meditate's will be the sprout.

Learn shows its graduation cap atop the rail, on the launch screen and on halkyon_app's
screens, as Todo and Train show theirs; the kingfisher leaves the app and stays the
F-Droid repository's icon.

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/lib/src/app/train_frame.dart

@@ -4,8 +4,8 @@ import 'package:halkyon_train_flutter/src/app/train_languages.dart';
 import 'package:halkyon_ui/halkyon_ui.dart';
 import 'package:material_ui/material_ui.dart';
 
-/// The app's icon: Lucide's `bike` on white (ADR 0031; still Todo's icon until
-/// Train's is drawn).
+/// The app's icon: Lucide's `dumbbell` on white, fading from cobalt into
+/// carrot, the app's backdrop (ADR 0031, D5 and D7).
 const trainIcon = AssetImage('assets/icon/halkyon_train_icon.png');
 
 /// Halkyon Train as halkyon_app's own screens know it (the first start, the