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.cs

@@ -166,6 +166,20 @@ internal sealed partial class TerminalScreen : UserControl
                 BorderThickness = new Thickness(1),
                 Foreground = Palette("TextDim"),
                 HorizontalContentAlignment = HorizontalAlignment.Center,
+
+                // ◆ NOT FOCUSABLE, AND THAT IS THE WHOLE CONTROL RATHER THAN A DETAIL.
+                //
+                // These keys are an extension of the keyboard, not a place the keyboard should go. As
+                // ordinary buttons they took Avalonia's focus on tap, which takes it off the NativeWebView
+                // — and the package's own OnLostFocus then calls the adapter's ResignFocus(). So pressing
+                // Tab or an arrow handed the terminal one byte and took the keyboard away from it: the next
+                // thing typed on a hardware keyboard went nowhere, and the row went on working because its
+                // buttons are pressed rather than typed into, which is what makes it look like the terminal
+                // had died instead.
+                //
+                // Focusable=false is what a toolbar button is, and it means the focused element never
+                // changes: the WebView is still it, so nothing resigns and nothing has to be handed back.
+                Focusable = false,
             };
 
             if (latches)