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-contrast.ts

@@ -62,7 +62,7 @@ export function deriveAdminPalette(
     warning: overrides.warning ?? defaults.warning,
     error: overrides.error ?? defaults.error,
     info: overrides.info ?? defaults.info,
-    sidebarText: readableColor(overrides.sidebar ?? defaults.sidebar, [overrides.canvas ?? defaults.canvas, overrides.surface ?? defaults.surface]),
+    sidebarText: readableColor(overrides.text ?? defaults.text, [overrides.sidebar ?? defaults.sidebar, overrides.canvas ?? defaults.canvas, overrides.surface ?? defaults.surface]),
     overlay: defaults.overlay,
     focusRing: accent,
   };

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};`,