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,
   };