a11y: add aria-current to active db-explorer pagination links

[xpoes123]

The tossup and bonus pagination in client/db/index.jsx was inaccessible to screen readers: the active page had no semantic marker, so assistive technologies announced all page links identically with no way to distinguish the current one.

Problem

In both the tossup and bonus pagination loops, the active page link was identified only by a CSS class:

<a className={`page-link ${isActive && 'active'}`} href='#' ...>

Two issues:

1. No aria-current attribute — screen readers cannot identify the current page.
2. isActive && 'active' evaluates to the boolean false (not an empty string) when inactive, injecting the literal text "false" into the className string.

Changes

• Adds aria-current={isActive ? 'page' : undefined} to the active page <a> in both the tossup (line 421) and bonus (line 480) pagination loops.
• Fixes the class expression to ${isActive ? 'active' : ''} to eliminate the spurious "false" class name.

Risk & testing

Two-line mechanical change per pagination component. The visual rendering is identical for sighted users; only the DOM attribute and class string differ. semistandard and node --check pass.