Let a group be right-clicked, as a host already can

The host cards have had a menu since the grid replaced the sidebar; the group
cards above them had a double-click and two buttons beside the heading, and
nothing that named the card under the pointer. Open, Edit and Delete are on them
now, drawn and aimed the same way.

On the list rather than in the item template, for the reason the host grid's is:
the three commands are the vault's, and a ContextMenu inside a DataTemplate has
the row for its data context, so every binding in it would silently resolve to
nothing. The code-behind selects whatever was right-clicked before the menu
opens, and that is what makes one menu act on the card under the pointer rather
than on whichever was selected before.

**Cancelled over the space around the cards, and here that guard is doing more
than the host grid's.** GroupTarget falls back to the group whose contents are on
screen when no card is selected — the right answer for a pair of buttons beside
the heading, which would otherwise have no subject the moment a group with
nothing inside it is opened, and the wrong one for a menu that opened on a card.
Without the guard, right-clicking the gap beside the cards would offer to delete
the group the trail ends with: a question about something the user is not
pointing at, in the one menu where the answer is a deletion.

Only Open takes a parameter, and it has to. OpenGroupCommand's null is a real
argument rather than a missing one — it is the trail's first crumb, ALL HOSTS —
so an entry with no parameter would not open the card, it would leave the group
the user right-clicked and go back to the top level.

Two tests beside the two the host menu already had. What they hold that a build
cannot is the CommandParameter binding: a path that resolves to nothing compiles
and draws, and the entry would then quietly do the opposite of what it says. The
popup itself is still the platform's, so manual-checks 7.9 gained the group half
of the same check.

docs/manual-checks.md

@@ -795,6 +795,16 @@ With host A selected, right-click host B and choose Delete.
 wrong machine. `HostGridTests` covers both halves headlessly, so this is a confirmation that a real popup
 behaves as the headless one did.
 
+**And the same on the group cards above.** Open a group, then right-click a card inside it and choose
+Delete.
+
+**Pass:** the question names the **card**, not the group that is open — and Open on that menu goes into the
+card, rather than back out to ALL HOSTS. Right-clicking the space around the group cards opens no menu.
+
+**Failure means:** the menu is reading `GroupTarget`'s fallback, which is the group whose contents are on
+screen. That fallback is right for the EDIT and DELETE buttons beside the heading and wrong for a menu that
+opened on a card.
+
 ### 7.10 Clicking a host in the palette connects
 
 Ctrl+K, then click a result with the mouse rather than pressing Enter.