Add install.sh / run.sh / bundled icon; fully pin requirements

- install.sh: one-time Linux Mint setup (apt, .venv, requirements, .desktop
  entry with absolute paths). Re-runnable.
- run.sh: launches the app from .venv (called by the menu entry).
- icon.png: bundled 256px red-cross launcher/window icon; app prefers it over
  the runtime-drawn fallback (data/icon.png).
- requirements.txt: fully pinned incl. transitive deps, tested on Python 3.12;
  numpy held at 2.2.6 so it installs on Python 3.10 too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

run.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# Startet die App aus ihrer virtuellen Umgebung.
+set -euo pipefail
+cd "$(dirname "$(readlink -f "$0")")"
+
+if [ ! -x .venv/bin/python ]; then
+    echo "Die App ist noch nicht eingerichtet. Bitte einmal  ./install.sh  ausführen." >&2
+    exit 1
+fi
+
+exec .venv/bin/python app.py