client/web: show manage button in readonly view

We render the readonly view in two situations:
- the client is in login mode, and the device is connected
- the client is in manage mode, but the user does not yet have a session

If the user is not authenticated, and they are not currently on the
Tailscale IP address, render a "Manage" button that will take them to
the Tailcale IP of the device and immediately start check mode.

Still to do is detecting if they have connectivity to the Tailscale IP,
and disabling the button if not.

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>
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