Simplify registration: direct users to Bookhoard web UI

Instead of showing a long auth URL that's impractical to type on
e-ink, tell users to open their Bookhoard web UI and approve from
Device Management. Uses persistent InfoMessage (no timeout) so
the message stays visible while they switch devices.

main.lua

@@ -510,11 +510,9 @@ function Bookhoard:startRegistration()
         end
 
         self.registration_id = result.registration_id
-        local auth_url = result.auth_url
 
         UIManager:show(InfoMessage:new{
-            text = T(_("Open this URL on your phone or computer to approve this device:\n\n%1\n\nWaiting for approval…"), auth_url),
-            timeout = 30,
+            text = T(_("Device registered on server.\n\nOpen your Bookhoard web UI, go to Device Management, and approve this device:\n\n%1\n\nWaiting for approval…"), self.settings.server_url),
         })
 
         self:startRegistrationPoll()