Add average pace (min/km) to summary cards

[c-wilkinson]

Extend the existing summary cards to include an average pace metric.

The new metric should:

• Show average pace in min/km for the period
• Be calculated consistently with existing distance and time totals
• Include comparison to the previous period, in line with other card metrics
• Follow the same visual and formatting conventions as existing summary card fields

This enhances the usefulness of the summary cards by adding an at-a-glance intensity indicator alongside volume-based metrics.