css: remove dark mode, unify token system, eliminate all hardcoded colors

- Remove @media (prefers-color-scheme: dark) block from variables.css
- Delete colors.css (dead reference doc, never loaded)
- Add semantic tokens to variables.css:
    --header-gradient-fade, --header-shadow-strong/soft, --header-nav-active-border
    --search-error-bg/border/color
    --sys-bg-surface/deep/panel, --sys-border/border-deep/border-hover
    --sys-text-dim/bright/body, --sys-overlay, --sys-syntax-* (7 highlight tokens)
    --success/warning/error/blue/yellow/green -muted-bg/border/hover alpha overlays
    --danger-border-muted
- Replace all hardcoded hex/rgba in common.css, main.css, search.css, admin.css, system.css
- Fix --border-color typo -> --border-primary in search.css
- Fix view-toggle__btn active color: --text-primary -> --accent-foreground
- Admin and public share identical token set, no separate admin theme
- Update README.md

public/assets/css/main.css

@@ -107,7 +107,7 @@
 }
 
 .card__gradient-author {
-  color: #ffffff;
+  color: var(--accent-foreground);
   font-size: 0.75rem;
   opacity: .85;
   margin-bottom: .25rem;
@@ -115,7 +115,7 @@
 }
 
 .card__gradient-title {
-  color: #ffffff;
+  color: var(--accent-foreground);
   font-size: 0.85rem;
   font-weight: 600;
   display: -webkit-box;