Add a dark and light mode toggle to the header

A sun and moon button at the end of the nav (and the Store's nav) swaps
the mode and remembers the choice in the browser. Until it is pressed the
site follows the device setting. A head script sets html[data-theme]
before first paint, so light styles now key off data-theme instead of a
media query, and motifs render both images with CSS showing one.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aSbo2D1qP8cMFQCykf7kx

services/site/src/pages/404.astro

@@ -83,11 +83,9 @@ const groups = [
     filter: drop-shadow(0 0 22px rgba(var(--gk-teal-rgb), 0.35));
   }
   /* Glow is for the night sky; on Starlight the foil stands on its own. */
-  @media (prefers-color-scheme: light) {
-    .lost :global(.seal) {
-      opacity: 1;
-      filter: none;
-    }
+  :global(:root[data-theme="light"]) .lost :global(.seal) {
+    opacity: 1;
+    filter: none;
   }
   .eyebrow {
     font-size: 11px;