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/0030-halkyon-train.md

@@ -224,7 +224,7 @@ Strava. What was found, 27 Oct:
   - **Android and iOS** record; **Linux, macOS and Windows** show and plan (D5); no web,
     as for Learn and Todo, whose keys (ADR 0017) need libsodium natively;
   - **Halkyon Train**, `app.halkyon.train`, in English and German alike;
-  - **its icon is Lucide's `bike`**, in the family of [ADR 0031](0031-the-apps-icons.md)
+  - **its icon is Lucide's `bike`** (Lucide's `dumbbell` since 2 Nov 2026, ADR 0031, D7), in the family of [ADR 0031](0031-the-apps-icons.md)
     ("activity" and "heart-pulse" look like basic health apps, "a defibrillator or ADAC
     health application"; "this is more about training"; Lucide has no runner);
   - **deployed once recording works** and the developer wants to use it, not as an