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>

apps/learn/halkyon_learn_flutter/linux/runner/my_application.cc

@@ -37,7 +37,7 @@ static void set_window_icon(GtkWindow* window) {
   g_autofree gchar* bundle = g_path_get_dirname(executable);
   g_autofree gchar* icon_path =
       g_build_filename(bundle, "data", "flutter_assets", "assets", "icon",
-                       "halkyon_icon.png", nullptr);
+                       "halkyon_learn_icon.png", nullptr);
   g_autoptr(GError) error = nullptr;
   if (!gtk_window_set_icon_from_file(window, icon_path, &error)) {
     g_warning("Couldn't load the window icon %s: %s", icon_path,