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/0031-the-apps-icons.md

@@ -2,7 +2,9 @@
 
 - **Status:** decided 28 Oct 2026, by the developer (D1–D4 below). Built the same day for
   Learn, Todo and Train by one tool, `tool/app_icons.py` (as *What follows* proposed, the
-  developer agreeing); Meditate's comes with Meditate.
+  developer agreeing); Meditate's comes with Meditate. **D5 (2 Nov 2026) changes D2's
+  gradients**: each icon ends in its app's backdrop colour; **D6** puts Learn's icon in
+  place of the kingfisher inside the app; **D7** gives Train the dumbbell, Meditate the sprout.
 - **Builds on:** ADR 0030, D12 (Train's icon is Lucide's `bike`).
 
 ## Context
@@ -56,3 +58,42 @@ through a colour between them (violet or sky) stays bright.
   can't draw. librsvg through PyGObject and pycairo (Ubuntu's `python3-gi`,
   `gir1.2-rsvg-2.0`, `python3-cairo`) renders them exactly; the previews were drawn so.
 - Lucide's licence (ISC) beside each icon and in each app's licence list, as Todo's is.
+
+## Changed 2 Nov 2026: each icon ends in its app's backdrop (the developer's)
+
+Trying Todo on 2 Nov, the developer found its icon, cobalt through pacific into fern,
+confusing beside its strawberry screens (ADR 0033, D3; D10 had kept the icon). Of the
+four, only Train's icon ended in its backdrop's colour (carrot); Learn's ended in
+strawberry, now Todo's, and at launcher size Todo's and Meditate's looked alike, as did
+Learn's and Train's. Compared as swatches the same day.
+
+- **D5 Every icon fades from cobalt into its app's backdrop colour**, the colour its
+  screens wash down from (`ScreenColor`), through a colour between as D1 has it. The
+  backdrops stay: Learn cobalt, Todo strawberry, Train carrot, Meditate fern (the
+  developer's, 2 Nov, over reef: a green). So:
+
+  | App | Glyph | Gradient |
+  |---|---|---|
+  | Learn | `graduation-cap` | cobalt `#3C57E5` → enzian `#5252CC` → purple `#8080FF` |
+  | Todo | `list-todo` | cobalt `#3C57E5` → violet `#8E4FD6` → strawberry `#EC3E7C` |
+  | Train | `dumbbell` (D7) | cobalt `#3C57E5` → violet `#8E4FD6` → strawberry `#EC3E7C` → carrot `#F78400` |
+  | Meditate | `sprout` (D7) | cobalt `#3C57E5` → pacific `#179FD9` → fern `#3CBB3A` |
+
+  Todo takes Learn's gradient, Meditate Todo's; Train's stays. Learn's backdrop is cobalt,
+  the family's start, so its icon stays in the blues, ending in purple (the agent's
+  recommendation, over cobalt → pacific → reef, which looked like Meditate's at launcher
+  size): its end colour lies furthest from the others' (strawberry, carrot, fern). The
+  pairs D1 kept change: Todo's and Train's are related now, Learn's and Meditate's stand
+  apart. Installed apps change their icon with their next release, as D3 says.
+
+- **D6 Learn shows its own icon inside the app** (the developer's, 2 Nov 2026): atop
+  the navigation rail, on the launch screen and on halkyon_app's screens (the first
+  start, signing in, the account switcher), where D3 had left the kingfisher, as Todo
+  and Train show theirs. The kingfisher (`halkyon_learn_flutter/assets/icon/halkyon_icon.png`,
+  removed) stays Halkyon's emblem where D3 put it otherwise: the F-Droid repository's
+  icon (`tool/release/fdroid/halkyon.png`).
+- **D7 Train's glyph is Lucide's `dumbbell`, Meditate's `sprout`** (the developer's,
+  2 Nov 2026), over D12's `bike` (ADR 0030) and D4's `circle-dot`: Train is more than
+  riding (runs, rows and strength, ADR 0030), and the sprout fits Meditate's
+  fern. The dumbbell lies from top right to bottom left, across the gradient, so the
+  carrot it ends in shows less than on the bike.