Wire the RA sort into the object list's Sort marking menu

[brickbots]

SortOrder.RA exists in the object list and, once #622 lands, sort() actually
implements it. The user still cannot select it — there is no marking-menu entry
that reaches it, so the sort is dead code.

Why it is unreachable

UIObjectList.__init__ builds the Sort submenu with only two live slots:

callback=MarkingMenu(
    up=MarkingMenuOption(),
    left=MarkingMenuOption(label=_("Nearest"), callback=self.mm_change_sort),
    down=MarkingMenuOption(),                      # <- bare, label="", callback=None
    right=MarkingMenuOption(label=_("Standard"), callback=self.mm_change_sort),
),

Two pieces of code already assume the down slot is RA and are therefore both
dead:

• __init__: if self.current_sort == SortOrder.RA: self.marking_menu.left.callback.down.selected = True
• mm_change_sort: if menu_item.label == _("RA"): ...

MarkingMenuOption() defaults to label="" and callback=None, so the slot
renders empty and dispatches nothing.

Fix

Give the down slot its label and callback:

down=MarkingMenuOption(label=_("RA"), callback=self.mm_change_sort),

That is the whole wiring — mm_change_sort and the selected bookkeeping are
already written for it, and #622 supplies the sort() branch and the
_sort_order_label entry.

i18n

None needed. msgid "RA" is already present and translated in all four
catalogues (de, es, fr, zh) — it has been used by the "Sorting by
{sort_order}" toast for some time.

Depends on

#622, which adds the SortOrder.RA branch to sort(). Wiring the menu without
it would give the user a menu entry that changes the label and nothing else.

Acceptance

• The Sort marking menu shows a third option, and choosing it orders the list by
  right ascension.
• The header and the in-list "Sort: …" label both read "RA" — both already route
  through _sort_order_label.
• Re-opening the list with RA active pre-selects the right marking-menu slot.
• Covered in tests/test_ui_modules.py the way the other sorts are.

Worth deciding while implementing

RA sorting is a plain sorted(..., key=lambda x: x.ra), so it cuts the sky at
RA 0 rather than at the meridian. That is the conventional catalog behaviour and
almost certainly what is wanted, but it is the sort of thing better settled
deliberately than by default.