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/data-model.md

@@ -66,7 +66,8 @@ generation, geocoding failures, and uncaught exceptions.
 and restored on start. Git-ignored, per-machine. Safe to delete (window opens at
 its default size).
 
-## `data/icon.png`
+## `icon.png` (repo root) and `data/icon.png`
 
-The red-cross window icon, drawn at first run and written here so the `.desktop`
-launcher can point at it. Git-ignored (regenerated). Delete to force a redraw.
+`icon.png` in the repo root is the **bundled** launcher/window icon (committed).
+`data/icon.png` is only a runtime-drawn fallback the app writes if the bundled
+one is missing — git-ignored.