Depth, light and translucency without changing the API

Niveus read flat: thin sheens, short shadows, opaque grey chrome and a
painted canvas. This retunes values and component internals only; every
class and token name is unchanged, so existing pages pick it up as is.

- Canvas: a faint accent light falls across the page, so translucent
  layers have something to show through.
- Surfaces: the surface sheen runs the full height (lit top, shaded
  foot); raised and filled sheens, highlights and bevels are stronger;
  the elevation scale is deeper and softer.
- Translucency: cards, dialogs, the sidebar and tab tracks let some of
  the canvas through, with blur so busy grounds never read as texture;
  frosted and glass materials are clearer with more blur.
- Filled buttons give off a soft glow of their own color.
- Light fields are paper instead of grey wells that looked disabled.
- Fixes: navigation bars wrap and lists scroll inside them instead of
  overflowing the page on phones; choice labels sit at control size;
  disabled ghost buttons no longer grow a filled well; the color-swatch
  rule is split so neither engine drops it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019iMUTHrD8K9CSuHqsanje9

niveus/components/window.css

@@ -37,6 +37,7 @@
   min-inline-size: 0;
 
   background-color: var(--nv-color-surface);
+  background-image: var(--nv-sheen-surface);
   color: var(--nv-color-text-primary);
   border: var(--nv-border-width) solid var(--nv-color-border-subtle);
   border-radius: var(--nv-radius-xl);