Add prefers-color-scheme dark mode for public pages

Scope: variables.css, search.css, todo/04-accessibility.md

- variables.css: add @media (prefers-color-scheme: dark) block scoped to
  body:not(.admin-body); overrides all semantic tokens with dark equivalents:
  --bg-* (#111→#333 range), --text-* (#eee/aaa/777),
  --border-* (#333/#444), --accent-primary lightened to #b87fd4
  (4.5:1 contrast on #111 background), --accent-secondary stays #9557b5,
  --accent-foreground flipped to #111111 for dark buttons,
  --accent-muted adjusted to rgba(184,127,212,0.15),
  status colours muted for dark (success #4db886, error #e05555,
  warning #d4a830); new --search-error-{bg,border,color} tokens added
  to :root (light: #fff0f0/#c00) and overridden in dark (#2a1515/#e05555)

- search.css: replace three hardcoded hex values in .search-error rule
  with var(--search-error-bg/border/color) so dark mode applies cleanly

- Admin pages are entirely unaffected: .admin-body body class is excluded
  from the dark-mode selector; system.css already has its own dark palette

public/assets/css/search.css

@@ -306,9 +306,9 @@
 
 /* Error message */
 .search-error {
-  background: #fff0f0;
-  border-left: 3px solid #c00;
-  color: #c00;
+  background: var(--search-error-bg);
+  border-left: 3px solid var(--search-error-border);
+  color: var(--search-error-color);
   padding: 0.5rem 1rem;
   font-size: 0.88rem;
   margin: 0.5rem 1.5rem;