Give the hosts board its vaults, and the Hetzner import a vault and a key

Six things, all of them answers to a question a screen was deciding on
somebody's behalf.

The board draws a heading per vault and sorts each vault's hosts by that
vault's own groups. A group is an item of one vault, so drawing every
group of every vault in one run flattened away containment that already
exists: two vaults each holding a "production" were two headings with the
same word on them, told apart by a small grey name beside each. Single-
vault sessions are unchanged. There is now one "No group" heading per
vault, so the folded set is keyed on SidebarGroupHeader.CollapseKey
rather than on GroupId ?? Guid.Empty, which had one vault's unfiled hosts
folding every other vault's away with them.

A Ctrl-click starts its set with the card the board has lit. The board
always has a selection — a fresh unlock lights the first host so CONNECT
has something under it — so "one card lit, nothing ticked" is the state
this screen opens in, and ticking a second card there produced a set of
one: the machine the user was looking at was left out of the count and
out of everything Delete, Move and Change group then ran over.

The card tooltip says the host's name as well as its address. 214 pixels
of card trims the name, and the tooltip was the one place it was not said
in full.

The card's floor drops from 120 to 104. A card with no tags measures
about 87, so the mock's number was thirty pixels of air under every card
on a board that is scrolled.

"Move to another vault…" joins the board's context menu for one host.
Moving one machine meant opening its pane and finding the menu inside it,
while moving six was a right click away — the one-host case was the
harder of the two.

And the Hetzner import asks which vault and which key. A project belongs
to a team more often than to a person, so forty servers landing wherever
the session happens to be standing is the wrong default; ImportHostsAsync
takes the target vault, and the ssh_config importer passes null and is
unchanged. The key is the half Hetzner cannot supply and the keychain
usually already holds — the public half went to Hetzner when the servers
were made — so the import points every host at it rather than bringing
one. It defaults to no key rather than guessing, and says so when the key
it was given lives in a different vault from the hosts.

src/DodoSSH.Client.App/Views/HostsScreen.axaml.cs

@@ -263,7 +263,9 @@ internal sealed partial class HostsScreen : UserControl
 
         if (control)
         {
-            vault.ToggleHostChoiceCommand.Execute(row);
+            // Extend rather than toggle: the set this starts has the lit card in it, which is the half a
+            // plain toggle left out. See VaultViewModel.ExtendHostChoice.
+            vault.ExtendHostChoiceCommand.Execute(row);
             anchor = row;
 
             Board.Focus();