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/overview.md

@@ -1,5 +1,8 @@
 # Overview – what the app does
 
+There is a **Hilfe** menu with "Nach Update suchen" (self-update via `git`, see
+[setup.md](setup.md#updating)) and "Version…".
+
 The app is a single window: a **DRK-red header bar**, two tabs, and a
 **colour-coded status bar** at the bottom (grey = neutral, green = success,
 orange = warning, red = error; transient messages fade back to neutral after