Add a dark theme, default, with a switch in the top bar

Colours move into theme tokens (light values in :root, dark in
[data-theme=dark]); the closing contact band inverts on each theme. The
choice is stored in localStorage and applied by a head script before the
first paint. Voxel covers and image shadows follow the theme.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XtcZtnG4GQA1d6mW5PNrS9

components/gizmo.module.css

@@ -8,8 +8,8 @@
   padding: 0;
   border: 0;
   border-radius: 50%;
-  background: rgba(244, 246, 248, 0.62);
-  box-shadow: 0 0 0 1px rgba(21, 24, 29, 0.08);
+  background: var(--disc-bg);
+  box-shadow: 0 0 0 1px var(--ring);
   -webkit-backdrop-filter: blur(8px);
   backdrop-filter: blur(8px);
   cursor: pointer;