Chart the maximum ASIC temperature by default

[0xjc65eth]

Summary

• make the default asicTemp chart source use the hottest ASIC temperature when a second sensor is present
• store historical asicTemp samples as the max of ASIC temp 1 and 2
• rename the chart label to Max ASIC Temp while keeping the existing asicTemp key compatible

Fixes #1476

Testing

• git diff --check
• npm run build

Note: the frontend build still reports the existing initial bundle budget warning.

[0xjc65eth]

Closing this for now to reduce maintainer review load, per feedback from @mutatrum. I am keeping #1705 open as the single focused PR because it is small, UI-only, and already has an ack. Happy to revisit this later only if maintainers ask for it. Thanks for the feedback and for maintaining the project.