Stop the terminal's accessory keys taking the keyboard off it

Ctrl, Esc, Tab, the arrows and the two text-size keys were ordinary Avalonia
buttons sitting over a NativeWebView. An ordinary button takes focus on tap,
which takes it off the WebView — and the package's own OnLostFocus then calls the
adapter's ResignFocus(). So pressing Tab handed the terminal one byte and took
the keyboard away from it, and everything typed afterwards went nowhere.

What makes it worth more than a one-line fix is the symptom. The row goes on
working, because its keys are pressed rather than typed into, so what you see is
a terminal that answers the buttons and ignores the keyboard — which reads as the
session having died rather than as anything to do with focus.

Focusable = false is what a toolbar button is: these keys are an extension of the
keyboard, not a place it should go. The focused element then never changes, so
nothing resigns and nothing has to be handed back — which matters, because the
hand-back is the direction platform-flags already records as the hard one.

The flags file gains the phone's half of that entry, and check 11.10 is the
measurement: this needs a paired hardware keyboard and there is no test on this
head that could stand in for one.

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

@@ -347,6 +347,11 @@
           Disabled at the ends rather than clamping silently. A button that keeps accepting taps and does
           nothing reads as the terminal having stopped responding, which is the one thing this screen must
           never look like.
+
+          ◆ Focusable="False", for the reason the accessory keys carry in code — see
+          TerminalScreen.axaml.cs. Every button on this row sits over a terminal somebody is typing into,
+          and an ordinary one takes Avalonia's focus off the NativeWebView on tap, which makes the package
+          resign the page's focus. Making the text smaller must not cost the keyboard.
         -->
         <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="5" Margin="8,0,12,0"
                     VerticalAlignment="Center">
@@ -356,7 +361,7 @@
           <Button Classes="row" MinHeight="30" Height="30" MinWidth="40" Padding="0" CornerRadius="9"
                   HorizontalContentAlignment="Center" VerticalContentAlignment="Center"
                   Background="{StaticResource Panel}" BorderBrush="{StaticResource BorderMid}"
-                  BorderThickness="1"
+                  BorderThickness="1" Focusable="False"
                   Command="{Binding ShrinkTerminalFontCommand}"
                   IsEnabled="{Binding CanShrinkTerminalFont}">
             <TextBlock Classes="mono" FontSize="13" Text="A−" />
@@ -365,7 +370,7 @@
           <Button Classes="row" MinHeight="30" Height="30" MinWidth="40" Padding="0" CornerRadius="9"
                   HorizontalContentAlignment="Center" VerticalContentAlignment="Center"
                   Background="{StaticResource Panel}" BorderBrush="{StaticResource BorderMid}"
-                  BorderThickness="1"
+                  BorderThickness="1" Focusable="False"
                   Command="{Binding EnlargeTerminalFontCommand}"
                   IsEnabled="{Binding CanEnlargeTerminalFont}">
             <TextBlock Classes="mono" FontSize="15" Text="A+" />