Add in-app self-update via git (Hilfe menu)

New Updater class + "Hilfe" menubar:
- "Nach Update suchen": git fetch + count of new upstream commits.
- Quiet background check on start; surfaces via status bar + menu label.
- Install: git merge --ff-only, pip install if requirements.txt changed,
  then restart via os.execv. data/ is git-ignored and untouched.
- Fast-forward only; diverged history or offline -> clear message, no action.
- Inert unless run from a git clone with git on PATH.

Also: "Version…" menu item shows the installed commit.

Verified against throwaway git repos (check / ff-update / diverged / no-op /
non-clone) and via GUI build on Python 3.14 / Tk 9.

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

docs/README.md

@@ -31,4 +31,4 @@ code) and plots them on an interactive map.
 - **UI language:** German; DRK-red header, enlarged font, colour-coded status bar, window position remembered
 - **Persistence:** atomic CSV writes + rotating backups in `data/backups/`; log at `data/app.log`
 - **Tests:** non-GUI smoke checks only (not yet a committed `tests/` suite)
-- **Version control:** git initialized 2026-09-07
+- **Version control:** git initialized 2026-09-07; in-app self-update (`git` fast-forward + restart) via the Hilfe menu