Add cone of uncertainty for Trio predictions

[bjorkert]

Summary

• Adds a "cone of uncertainty" visualization for Trio predictions, matching Trio's native forecast display. Instead of four individual prediction lines (ZT, IOB, COB, UAM), a filled band shows the min/max envelope across all prediction arrays at each 5-minute time step.
• Adds a Prediction Style setting (Cone/Lines) for Trio users. Loop users are locked to its single prediction line with the picker hidden.
• Reactively monitors device type changes via Combine to auto-set the appropriate default (cone for Trio, lines for Loop) while preserving manual user preference across Nightscout refreshes.