feat(top): add vertical pagination to statistics views

[zbinorama]

Summary

Large statistics views in pgcenter top could not be fully inspected when
the result contained more rows than fit in the terminal.

This change adds vertical pagination using PgUp and PgDn:

• keeps the table header visible while paging;
• preserves the current snapshot during redraws;
• clamps the vertical offset after filtering, refresh, and terminal resize;
• preserves horizontal column scrolling;
• resets the vertical position when switching views or changing sorting/filtering;
• adds tests for pagination, filtering, redraws, header rendering, and scroll offsets.

Validation

• make test
• make build
• Manual TUI verification with PostgreSQL fixtures on port 21917
• Verified PgDn → PgDn → PgUp with a result exceeding terminal height

Follow-up

Horizontal scrolling remains on the existing viewCh redraw path. It is
outside the scope of this change, but could be migrated to the dedicated
redrawCh path in a follow-up change.

AI assistance: OpenAI Codex was used to assist with the implementation and tests. The changes were reviewed and validated by the author.