Fix slideshow controls alignment

[MattNotarangelo]

Summary

Moved from RopeWiki/commonjs#16 per review feedback — CSS belongs here, not injected via JS.

• Center page number readout by adding left: 0 (overrides default -0.3em offset)
• Align slider with pause button by zeroing asymmetric vertical margins (-11px top, 7px bottom → 0)
• Round all corners of pause button evenly (border-radius: 4px) instead of only the right side

Test plan

Console command to preview all three fixes:

var s = document.createElement('style');
s.innerHTML = '.srf-slideshow .slideshow-nav-readout { left: 0; } .srf-slideshow .slideshow-nav { margin-top: 0; margin-bottom: 0; } .srf-slideshow .slideshow-nav-wrapper .ui-button-icon-only { border-radius: 4px; }';
document.body.appendChild(s);

[hcooper]

I've pushed this to production - looks good.