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/search.css

@@ -36,7 +36,7 @@
 
   .repertoire-col {
     border-right: none;
-    border-bottom: 1px solid var(--border-color);
+    border-bottom: 1px solid var(--border-primary);
     padding: 1rem 0 1.25rem;
   }
 
@@ -191,7 +191,7 @@
 .view-toggle__btn.active,
 .view-toggle__btn:hover {
   background: var(--accent-primary);
-  color: var(--text-primary);
+  color: var(--accent-foreground);
 }
 
 /* Search controls bar */