feat(learn,todo,train,meditate): the apps' icons on a dark tile (ADR 0031, D8)

Each icon's Lucide glyph now sits on the apps' dark background (#131520)
instead of white, its cobalt-to-colour gradient lifted 30 % towards white
so the glyph keeps above 4.5 : 1 against the tile. tool/app_icons.py
holds the tile and the lift; the pubspecs give Android's adaptive icon
and iOS's the same background. Every platform's icon is regenerated.

Whats-New: The app's icon now sits on a dark background.
Whats-New-De: Das App-Symbol hat jetzt einen dunklen Hintergrund.
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/adr/0031-the-apps-icons.md

@@ -4,7 +4,8 @@
   Learn, Todo and Train by one tool, `tool/app_icons.py` (as *What follows* proposed, the
   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.
+  place of the kingfisher inside the app; **D7** gives Train the dumbbell, Meditate the sprout;
+  **D8 (7 Nov 2026) changes D1's white tile** to the apps' dark background, the gradients lifted.
 - **Builds on:** ADR 0030, D12 (Train's icon is Lucide's `bike`).
 
 ## Context
@@ -97,3 +98,17 @@ Learn's and Train's. Compared as swatches the same day.
   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.
+
+- **D8 Every icon's tile is the apps' dark background, `#131520`, its gradient lifted 30 %
+  towards white** (the developer's, 7 Nov 2026, on the recommendation among four dark
+  tiles shown side by side, over D1's white): halkyon_ui's dark surface, so the icon,
+  the launch screen and the apps in dark mode are one surface. On that tile D5's colours
+  as they are sink in where they start (cobalt reaches about 2.5 : 1 against it); each
+  stop mixed 30 % towards white keeps its hue and holds the darkest of them above
+  4.5 : 1 (cobalt 5.7, enzian 5.6), so the glyphs read at launcher size on any wallpaper.
+  The other tiles shown: the gradients unchanged, a tile fading from the dark theme's
+  cobalt container into near-black, and black, which vanishes on dark wallpapers. The
+  palette stays D5's, Proton's label colours; `tool/app_icons.py` holds the tile and the
+  lift (`TILE`, `LIFT`), and the apps' pubspecs give Android's adaptive background and
+  iOS's the tile's colour. Inside the apps (D6) the icon is the dark tile in light mode
+  too, as on the launcher. Installed apps change their icon with their next release.