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>

tool/app_icons.py

@@ -1,7 +1,9 @@
 #!/usr/bin/env python3
-"""Makes the apps' icons, one family (ADR 0031): each app's Lucide glyph on a white
-square, fading from cobalt through the colours APPS names, top left to bottom right
-across Lucide's 24-unit box, the stops evenly spaced, at Lucide's stroke width (2),
+"""Makes the apps' icons, one family (ADR 0031): each app's Lucide glyph on a dark
+square (TILE, the apps' dark background; D8), fading from cobalt through the colours
+APPS names, each lifted towards white (LIFT) so the glyph stands out on the dark tile,
+top left to bottom right across Lucide's 24-unit box, the stops evenly spaced, at
+Lucide's stroke width (2),
 1024 x 1024 pixels (<app>/<flutter package>/assets/icon/<icon>); the launchers and
 the launch screens round its corners themselves. Then, for each app:
 
@@ -9,7 +11,7 @@ the launch screens round its corners themselves. Then, for each app:
   app registers in its licence list;
 - the snap's icon where the app has a snap (Learn's snap/gui/halkyon-learn.png at the
   repository's root, Todo's apps/todo/snap/gui/halkyon-todo.png; 512 x 512 pixels, the
-  Snap Store's size), the icon with rounded corners, as other apps' white tiles
+  Snap Store's size), the icon with rounded corners, as other apps' tiles
   have (radius 142.222 of 512, about 28 %);
 - through flutter_launcher_icons, configured in the app's pubspec: Android's adaptive
   icon and its older square one, iOS's, macOS's (the rounded tile on Apple's grid,
@@ -59,6 +61,12 @@ REEF = "#1DA583"
 ENZIAN = "#5252CC"
 PURPLE = "#8080FF"
 
+# ADR 0031, D8: the tile is halkyon_ui's dark surface, and every stop is mixed this
+# share towards white, so the gradient's darkest part (cobalt) keeps above 4.5 : 1
+# against it.
+TILE = "#131520"
+LIFT = 0.3
+
 
 @dataclass(frozen=True)
 class AppIcon:
@@ -130,8 +138,13 @@ def rgb(colour: str) -> tuple[float, float, float]:
     return tuple(int(colour[i : i + 2], 16) / 255 for i in (1, 3, 5))
 
 
+def lifted(colour: str) -> tuple[float, float, float]:
+    """[colour] mixed LIFT towards white."""
+    return tuple(c + (1 - c) * LIFT for c in rgb(colour))
+
+
 def icon(glyph: str, stops: tuple[str, ...], svg: str) -> Image.Image:
-    """[glyph] on white in the gradient of [stops], SIZE pixels square."""
+    """[glyph] on TILE in the lifted gradient of [stops], SIZE pixels square."""
     if f"lucide-static v{LUCIDE}" not in svg:
         raise SystemExit(f"Not lucide-static {LUCIDE}'s {glyph}.")
     if 'viewBox="0 0 24 24"' not in svg or 'stroke-width="2"' not in svg:
@@ -150,11 +163,11 @@ def icon(glyph: str, stops: tuple[str, ...], svg: str) -> Image.Image:
 
     tile = cairo.ImageSurface(cairo.FORMAT_RGB24, SIZE, SIZE)
     context = cairo.Context(tile)
-    context.set_source_rgb(1, 1, 1)
+    context.set_source_rgb(*rgb(TILE))
     context.paint()
     gradient = cairo.LinearGradient(offset, offset, offset + box, offset + box)
     for i, colour in enumerate(stops):
-        gradient.add_color_stop_rgb(i / (len(stops) - 1), *rgb(colour))
+        gradient.add_color_stop_rgb(i / (len(stops) - 1), *lifted(colour))
     context.set_source(gradient)
     context.mask_surface(mask)