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

todo/04-accessibility.md

@@ -76,4 +76,4 @@
 
 ## 5 - Motion & user preferences
 
-- [ ] **`prefers-color-scheme` not respected** — site has fixed white public / fixed dark admin themes; consider a dark-mode variant for public pages (not a WCAG failure, but quality-of-life)
+- [x] **`prefers-color-scheme` not respected** — `variables.css` now includes a `@media (prefers-color-scheme: dark)` block scoped to `body:not(.admin-body)`; overrides all semantic tokens (`--bg-*`, `--text-*`, `--border-*`, `--accent-*`, status colours) with dark equivalents; `--accent-primary` lightened to `#b87fd4` for 4.5:1 contrast on dark backgrounds; `search.css` `.search-error` hardcoded `#fff0f0`/`#c00` replaced with `--search-error-bg`/`--search-error-border`/`--search-error-color` variables also overridden in dark mode; admin pages unaffected (`.admin-body` preserves light-mode values)