refactor the mobile UI

[devoncarew]

Refactor the mobile UI:

• move from using three tabs (code, ui, console) to two (code and app output)
• when starting to run an app, auto-switch to the app output tab
• fix https://github.com/dart-lang/dart-pad/issues/2859

This should address an issue where the execution iframe wasn'y inited when we tried to run something. It also combines the app + console tabs into one (as we do on the regular desktop UI).

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