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>

docs/adr/0033-halkyon-todo.md

@@ -186,6 +186,8 @@ the rail, a wash of strawberry, settings grouped as theirs. What the screenshots
   app's wash and its icon already differ, and Todo's and Meditate's icons are kept
   related (ADR 0031, D1). The agent made no recommendation; this is the default the
   decision of 1 Nov left in place, to be asked again if the screens look wrong beside it.
+  Asked again on 2 Nov: changed by ADR 0031, D5, the icon now fading from cobalt
+  through violet into strawberry, its backdrop.
 
 ## Decisions after trying it (1 Nov 2026, the developer's)