Keep the phone's nav under Connections when nothing is running

The chrome stands down for a shell, and it was standing down for the whole
terminal surface. Those parted company when that surface gained a connect page:
with no tabs open it draws a box, a CONNECT button and the machines connected to
before, which is a page in everything but which enum it is in. A third of the
display is worth giving to a shell and is not worth giving to that. Worse, it is
the one screen somebody arrives at by closing their last tab — so the state the
collapsed bar was most likely to be seen in was the state where it left the
system back gesture as the only route to Hosts or Settings.

So RefreshChrome reads one more question. IsTerminalSurface with no tabs joins
the pages in both flags, which keeps the rail and the bar in step: above 600dp
the rail is the bar, and fixing only the narrow layout would leave an unfolded
device on the same screen with the same nothing. The vault header is deliberately
not part of it. The surface draws its own bar with back and the +, and a header
above that is the second row of chrome this head exists to avoid.

The Connections entry lights for the first time, on IsTerminalSurface. It was
left unbound on the argument that the bar was never drawn while that surface was
up, so a lit state was unreachable — that argument is now false, and the flag is
unambiguous on a control that is only drawn in two situations: false on every
page, true on the connect page, and never read while a shell is showing. A bar
sitting under a screen it does not point at is the entry looking broken instead.

Nothing here is testable on this head — the phone's rectangles have no coverage,
for the reasons Phase 8 of manual-checks records — so 11.7 gains the check that
the bar is there with Connections lit, and 11.1 keeps the one that it is gone
with a shell up, which is the half that pays for the arrangement.

src/DodoSSH.Client.Android/Views/PhoneRail.axaml

@@ -134,15 +134,17 @@
 
           <!--
             The terminal is a surface rather than a page, so it goes through its own command — the same
-            reason the bottom bar's middle entry does. It is never lit, and deliberately: this rail is
-            collapsed while a shell is showing, so IsCurrent could only ever read false. What marks that
-            destination as current is the surface filling the screen.
+            reason the bottom bar's middle entry does. It lights on IsTerminalSurface, which on this control
+            can only mean one thing: the rail stands down for a shell, so the one moment it is drawn beside
+            this surface is Connections with nothing running. That screen is a page in everything but which
+            enum it is in, and a rail sitting beside it with no entry lit would be pointing nowhere.
 
             No count, unlike the two above it. The shells strip sits above the bottom of every page on this
             surface and is that count, drawn as the sessions themselves — a number beside this word would be
             the same fact said twice, three inches apart.
           -->
-          <Button Classes="railentry" Command="{Binding ShowTerminalCommand}">
+          <Button Classes="railentry" Classes.active="{Binding IsTerminalSurface}"
+                  Command="{Binding ShowTerminalCommand}">
             <Grid ColumnDefinitions="Auto,*,Auto">
               <TextBlock Grid.Column="0" Classes="railglyph" Text="⌗" />
               <TextBlock Grid.Column="1" Classes="raillabel" Text="Connections" />