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.

docs/manual-checks.md

@@ -1334,6 +1334,23 @@ by gesture or by the arrow, returns to Settings and not to HOSTS; a second back
 **Failure means:** `ShellScreen.Vault` is missing from `IsMoreSurface` or from the back gesture's first
 case, and those two have to move together — the switch mirrors that property by construction.
 
+### 11.10 The accessory keys do not cost the terminal its keyboard · **needs a hardware keyboard**
+
+With a shell open and a Bluetooth or USB keyboard paired, type into the terminal and confirm it arrives.
+Now tap **Tab** on the accessory row, or **A+**, and type again.
+
+**Pass:** the byte the key sent arrives, and so does everything typed after it. The terminal still has the
+keyboard.
+
+**Failure means:** the buttons on that row have become focusable again. An ordinary Avalonia button takes
+focus on tap, which takes it off the `NativeWebView` — and the package's own `OnLostFocus` then resigns the
+page's focus. The row goes on working, because its keys are pressed rather than typed into, so the symptom
+is a terminal that answers the buttons and ignores the keyboard: it reads as the session having died. See
+`Focusable = false` in `TerminalScreen.axaml.cs` and the focus entry in `docs/platform-flags.md`.
+
+Worth doing on the software keyboard too, where the same fault shows as the keyboard closing on the first
+tap of an arrow key.
+
 ---
 
 ## Phase 12 — Shared vaults: the operations that span two accounts