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/common.css

@@ -48,7 +48,7 @@ header {
     var(--gradient-3) 40%,
     var(--gradient-4) 60%,
     var(--gradient-4) 88%,
-    rgba(149, 87, 181, 0) 96%
+    var(--header-gradient-fade) 96%
   );
 }
 
@@ -91,7 +91,7 @@ header nav ul a {
 /* Subtle shadow on all header text to improve legibility against the gradient */
 header nav > a,
 header nav ul a {
-  text-shadow: 0 0 16px rgba(119, 70, 145, 1), 0 0 32px rgba(119, 70, 145, 0.8);
+  text-shadow: 0 0 16px var(--header-shadow-strong), 0 0 32px var(--header-shadow-soft);
 }
 
 header nav ul a:hover {
@@ -100,7 +100,7 @@ header nav ul a:hover {
 
 header nav ul a[aria-current="page"] {
   opacity: 1;
-  border-bottom: 1px solid rgba(255, 255, 255, 0.6);
+  border-bottom: 1px solid var(--header-nav-active-border);
   padding-bottom: 1px;
 }