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>

docs/adr/0031-the-apps-icons.md

@@ -1,8 +1,8 @@
 # ADR 0031: The apps' icons, one family
 
-- **Status:** decided 28 Oct 2026, by the developer (D1–D4 below). Not built yet: Train's
-  icon comes with its shell (ADR 0030, D12), Learn's and Todo's follow, Meditate's with
-  Meditate.
+- **Status:** decided 28 Oct 2026, by the developer (D1–D4 below). Built the same day for
+  Learn, Todo and Train by one tool, `tool/app_icons.py` (as *What follows* proposed, the
+  developer agreeing); Meditate's comes with Meditate.
 - **Builds on:** ADR 0030, D12 (Train's icon is Lucide's `bike`).
 
 ## Context