assets: fix logo alignment and error icon centering

Tighten the SVG viewBox to the actual content bounding box and
remove hardcoded width/height attributes so the browser no longer
adds horizontal padding via preserveAspectRatio. The "h" wordmark
now left-aligns with the page content below it.

Replace the error icon SVG path (which had an off-center X) with
a simple circle + two crossed lines drawn from a centered viewBox.
Both icons now use fill="currentColor" for dark mode adaptation.

Updates juanfont/headscale#3182