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>

apps/todo/halkyon_todo_flutter/android/app/src/main/res/values/colors.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <color name="ic_launcher_background">#ffffff</color>
+    <color name="ic_launcher_background">#131520</color>
 </resources>