fix: remove double temp conversion — segment display just shows pre-converted values

HardwareMetrics.with_temp_unit converts once (controller). Segment display
(view) was converting again via display_temp(). Now _to_display_temp just
truncates to int. Removed dead display_temp() from models.py.

MVC: model has raw data, controller converts once, view displays.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>