TextViewer: Flag as a modal (#7255)

It's used that way in a few contexts (e.g., View HTML).

Fix #7249

frontend/ui/widget/textviewer.lua

@@ -33,6 +33,7 @@ local _ = require("gettext")
 local Screen = Device.screen
 
 local TextViewer = InputContainer:new{
+    modal = true,
     title = nil,
     text = nil,
     width = nil,