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/meditate/halkyon_meditate_flutter/pubspec.yaml

@@ -103,11 +103,11 @@ flutter:
 flutter_launcher_icons:
   image_path: "assets/icon/halkyon_meditate_icon.png"
   android: true
-  adaptive_icon_background: "#ffffff"
+  adaptive_icon_background: "#131520"
   adaptive_icon_foreground: "assets/icon/halkyon_meditate_icon.png"
   ios: true
   remove_alpha_ios: true
-  background_color_ios: "#ffffff"
+  background_color_ios: "#131520"
   macos:
     generate: true
     image_path: "build/icons/macos.png"