v0.6.26

fix: menu dropdown overlays the dashboard instead of blanking the screen

The menu bar opened each dropdown as a transparent ModalScreen. Textual does
not composite the screen below it here, so the whole dashboard went black
behind the dropdown. Replace the modal with an absolutely-positioned overlay
(MenuDropdown) mounted on the same screen, on a dedicated 'menu' layer — the
dashboard stays fully visible underneath. The app tracks the open dropdown:
clicking a title toggles it, clicking an item runs the action then closes,
Escape closes, and any other click (bubbling to App.on_click) closes it.

Bumps to 0.6.26.