Site: actually center the Get-Started difficulty cards

The previous attempt added `justify-content: center` to .difficulty-grid
but the cards stayed pinned to the left of the section. Cause: the
shorthand `margin: 20px 0 32px` on the grid set both horizontal
margins to 0, silently overriding the `margin-left: auto;
margin-right: auto` from the `.section-alt > *` rule that does the
section-wide centering. So the grid was 1180 px wide pinned to the
section's left content edge instead of being centered, and `justify-
content: center` only centered the four cards inside an
already-left-aligned grid container.

Fix: replace the shorthand with `margin-top: 20px; margin-bottom: 32px`
so the inherited auto horizontal margins survive. The other grids on
this page (.feature-grid, .parents-grid, .pet-cards) already use only
`margin-top` for the same reason — this brings .difficulty-grid in
line with that pattern.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Marcel Dütscher

site/styles.css

@@ -594,7 +594,13 @@ pre code { background: none; padding: 0; color: inherit; }
      as "left-aligned" against the narrower h2/section-lead above. */
   grid-template-columns: repeat(auto-fit, minmax(220px, 260px));
   gap: 14px;
-  margin: 20px 0 32px;
+  /* Keep the auto left/right margins from `.section-alt > *` so the grid
+     sits centered in the section. The earlier `margin: 20px 0 32px`
+     shorthand silently zeroed the horizontal margins and pinned the grid
+     to the left content edge — which is exactly what produced the visible
+     left-shift the user reported on a wide viewport. */
+  margin-top: 20px;
+  margin-bottom: 32px;
   justify-content: center;
 }
 .difficulty-card {