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;

public/assets/css/variables.css

@@ -2,6 +2,7 @@
    CSS VARIABLES (CUSTOM PROPERTIES)
    ============================================================ */
 
+/* ── Light mode (default) ─────────────────────────────────────────────── */
 :root {
   --bg-primary: #ffffff;
   --bg-secondary: #f5f5f5;
@@ -27,4 +28,42 @@
   --gradient-2: #60ECB4;
   --gradient-3: #E390FF;
   --gradient-4: #9557B5;
+
+  /* Search error block (public only, overridden in dark mode) */
+  --search-error-bg: #fff0f0;
+  --search-error-border: #c00;
+  --search-error-color: #c00;
+}
+
+/* ── Dark mode — public pages only (.admin-body keeps light vars) ─────── */
+@media (prefers-color-scheme: dark) {
+  body:not(.admin-body) {
+    --bg-primary:    #111111;
+    --bg-secondary:  #1a1a1a;
+    --bg-tertiary:   #252525;
+    --bg-active:     #333333;
+
+    --text-primary:   #eeeeee;
+    --text-secondary: #aaaaaa;
+    --text-tertiary:  #777777;
+
+    --border-primary:   #333333;
+    --border-secondary: #444444;
+
+    /* Accent hue lightened for contrast on dark bg (4.5:1 against #111) */
+    --accent-primary:    #b87fd4;
+    --accent-secondary:  #9557b5;
+    --accent-foreground: #111111;
+    --accent-muted:      rgba(184, 127, 212, 0.15);
+
+    /* Status colours — slightly muted on dark */
+    --success: #4db886;
+    --error:   #e05555;
+    --warning: #d4a830;
+
+    /* Search error block */
+    --search-error-bg:     #2a1515;
+    --search-error-border: #e05555;
+    --search-error-color:  #e05555;
+  }
 }