fix(mtext): stop Up/Down walking command history under the editor

The MText editor paints its own caret and is not a focusable widget, so
the command input keeps widget focus behind the dialog. It captures the
arrow itself, which routes the key through CommandLineArrowProbe instead
of ArrowKeyPressed — and that path only asked whether the command input
was focused, never whether an editor was open. Up and Down therefore
walked command history while the user was typing in MText.

Give the probe the same rule ArrowKeyPressed already applies: while the
editor is open the arrow moves its caret. The probe now carries the shift
state so a held Shift still extends the selection on this path.

Fixes #655

src/app/view/mod.rs

@@ -2241,6 +2241,7 @@ impl OpenCADStudio {
                         {
                             return Some(Message::CommandLineArrowProbe {
                                 direction: arrow?,
+                                extend_selection: modifiers.shift(),
                             });
                         }
                         // A focused web text field needs the browser clipboard;