Fix right-click context menus firing their first item on Linux

Bound to <Button-3> with finally: menu.grab_release(), the menu lost its grab
immediately and the trailing <ButtonRelease-3> activated the entry under the
cursor — right-clicking a history row jumped straight into the edit dialog and
deleting was impossible.

- RIGHT_CLICK constant: <ButtonRelease-3> (<ButtonRelease-2> on macOS)
- history + queue menus built once on self, not per click
- drop the premature grab_release()

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

docs/overview.md

@@ -26,7 +26,7 @@ then save the whole batch at once.
    as the background geocoder works through the queue.
    - Select a row and press <kbd>Delete</kbd>, use the **"Auswahl entfernen"**
      button, or click the `✕` cell.
-   - Right-click a row for *Löschen* / *Koordinaten erneut suchen*.
+   - Right-click a row for *Entfernen* / *Koordinaten erneut suchen*.
 3. **Action bar**
    - **Alle speichern** – appends every queued row to `orte.csv`. If any row
      looks like a duplicate (same date + city + PLZ), a **modal** lists them and