UI: Fix crash on macOS when closing OAUTH browser panel

Deleting the cefWidget in the panel's destructor can result in a crash
on macOS because the underlying NSView might have been destroyed in
memory between Qt closing the QDialog and the destructor call.

Moving the browser destruction into the accept and reject calls ensures
that CEF can clean up its state _before_ Qt tears down the view
hierarchy.
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Internal Server Error - Gitea: Git with a cup of tea
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