Query MySQL only once for Uptime.

[jfg956]

When running ps-top on an instance with general log enabled, I see the three (3) queries below being executed every second.

2026-01-19T19:14:20.894785-00:00          187 Prepare   SELECT VARIABLE_VALUE FROM performance_schema.global_status WHERE VARIABLE_NAME = ?
2026-01-19T19:14:20.894842-00:00          187 Execute   SELECT VARIABLE_VALUE FROM performance_schema.global_status WHERE VARIABLE_NAME = 'Uptime'
2026-01-19T19:14:20.895481-00:00          187 Close stmt

I trace this down to below:

• https://github.com/sjmudd/ps-top/blob/v1.1.16/display/display.go#L78
• https://github.com/sjmudd/ps-top/blob/v1.1.16/config/config.go#L53

In the grand scheme of things, running these 3 queries every second is not such a problem as the associated load on the database is probably negligible. On the other hand, this increases the number of queries needed every second by ps-top from 1 to 4, being 75% of the total number of queries run every second by ps-top. Moreover, these are not needed, as querying Uptime once should be enough to compute it client-side.

This PR removes the need for these 3 queries every second, querying Uptime only once, and then computing it client side.