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>

docs/changelog.md

@@ -2,6 +2,16 @@
 
 Notable changes to the app. Newest first.
 
+## 2026-09-07 — one-command install + launcher
+
+- `install.sh` – one-time Linux Mint setup: apt packages, `.venv` +
+  `requirements.txt`, and a `.desktop` menu entry with correct absolute paths.
+- `run.sh` – launches the app from `.venv`; the menu entry calls this.
+- `icon.png` – bundled red-cross launcher/window icon (256×256); the app now
+  prefers this over the runtime-drawn one.
+- `requirements.txt` – **fully pinned** (direct + transitive), tested on
+  Python 3.12. `numpy` capped at 2.2.6 so it still installs on Python 3.10.
+
 ## 2026-09-07 — built-in self-update
 
 New **Hilfe** menu: