feat: the apps' icons as one family, drawn by one tool (ADR 0031)

tool/app_icons.py at the root replaces Learn's and Todo's scripts: each
app's Lucide glyph (lucide-static 1.50.0) drawn by librsvg, fading from
cobalt through the colours ADR 0031 names, then every platform's
launcher icon through flutter_launcher_icons, Windows' .ico with every
size, and Learn's snap icon.

Learn's icon becomes the graduation cap, its window's on Linux too, with
Lucide's notice in its licence list; the kingfisher stays the emblem on
the rail and the launch screen. Todo's becomes list-todo, Train's bike.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

snap/snapcraft.yaml

@@ -45,7 +45,7 @@ website: https://halkyon.app/learn
 # though snapcraft's reference allows mail addresses there (canonical/snapcraft#6062
 # asks for them). Reports come by mail until people can open an account on the issue
 # tracker, so the address is in the description.
-# The app's icon with rounded corners, made by tool/app_icons.py; also the desktop file's.
+# The app's icon with rounded corners, made by the repository's tool/app_icons.py; also the desktop file's.
 icon: snap/gui/halkyon-learn.png
 grade: stable
 confinement: strict