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It's been done this way so you can still access the entire system from your phone or tablet, rather than only a small subset of it via a specific app.
To use the system on your phone, just type in the same address as your normally would use on your desktop or laptop computer, enter your username & password and you should be able to use everything as normal.
Note that administrative areas are designed to be used on larger screens, and may not function in the same way when viewed on a smaller screen.
Adding an icon to your home-screen
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