Center the content column at the docs' measure

On wide panes the 1fr center cell stretched, so content hugged the left
rail with dead space before the TOC. The main column now caps at the
docs' ~760px measure and centers in its cell, matching how the docs
balance a wide viewport.

Author: Sg312

apps/sim/lib/workspace-files/artifact-stylesheet.ts

@@ -475,7 +475,16 @@ figcaption { font-size: 0.875em; line-height: 1.4285714; color: var(--text-prima
 }
 .art-theme:hover { background: var(--surface-active); }
 .art-cols { display: grid; gap: 2.25rem; grid-template-columns: 1fr; }
-.art-cols > .art-main { padding-top: 1.5rem; min-width: 0; }
+.art-cols > .art-main {
+  padding-top: 1.5rem;
+  min-width: 0;
+  /* The docs' content measure, CENTERED in its cell: 1400px container minus
+     the rails leaves ~760px of prose column; without the cap the content
+     hugged the left rail and left dead space before the TOC on wide panes. */
+  width: 100%;
+  max-width: 760px;
+  margin: 0 auto;
+}
 /* Rails stagger with the viewport the way the docs do on a laptop: the
    section sidebar survives down to resource-panel widths, the TOC joins on
    wide surfaces, and a truly narrow pane collapses to a single column. The