Fix HK sidebar menu text readability

- Derive sidebar text color from the main admin text against the actual
  sidebar background (not canvas/surface), guaranteeing contrast
- Brighten muted sidebar text to 82% of the readable text color so
  inactive nav items and section labels stay clearly visible

src/lib/theme-css.ts

@@ -34,7 +34,7 @@ export function adminPaletteCss(admin: AdminPalette): string {
   const parts: string[] = [
     `--admin-sidebar-background:${admin.sidebar};`,
     `--admin-sidebar-text-readable:${admin.sidebarText};`,
-    `--admin-sidebar-muted-readable:color-mix(in srgb, ${admin.sidebarText}, ${admin.sidebar} 38%);`,
+    `--admin-sidebar-muted-readable:color-mix(in srgb, var(--admin-sidebar-text-readable) 82%, transparent);`,
     `--admin-input-text-readable:${readableColor(admin.text, [admin.canvas, admin.surface])};`,
     `--admin-canvas:${admin.canvas};`,
     `--admin-surface:${admin.surface};`,