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/learn/halkyon_learn_flutter/pubspec.yaml

@@ -194,10 +194,8 @@ flutter:
     # (<version>.<language>.txt), drafted by tool/release.dart and edited
     # (Halkyon Train's ADR 0030, D61): halkyon_app's What's new shows them.
     - assets/whats_new/
-    # The kingfisher, Halkyon's emblem: atop the navigation rail and on the launch
-    # screen.
-    - assets/icon/halkyon_icon.png
-    # The app's icon (ADR 0031), the window's on Linux, and Lucide's notice for it.
+    # The app's icon (ADR 0031), the window's on Linux, atop the navigation rail and
+    # on the launch screen (D6), and Lucide's notice for it.
     - assets/icon/halkyon_learn_icon.png
     - assets/icon/LICENSE-lucide.txt
     # Region maps for map questions (see assets/maps/ATTRIBUTION.md).