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/base.css

@@ -24,6 +24,14 @@ body {
   margin: 0;
   min-height: 100svh;
   background-color: var(--nv-color-background);
+  /* The canvas is lit, not painted: a wash of the accent falls in from the top
+     corner and a fainter one pools at the opposite foot. It is what gives
+     translucent surfaces something to be translucent against, and keeps a
+     page of solid panels from reading as one flat field. */
+  background-image:
+    radial-gradient(120% 80% at 0% 0%, color-mix(in oklab, var(--nv-color-accent) light-dark(10%, 16%), transparent), transparent 60%),
+    radial-gradient(90% 70% at 100% 100%, color-mix(in oklab, var(--nv-color-accent) light-dark(6%, 10%), transparent), transparent 60%);
+  background-attachment: fixed;
   color: var(--nv-color-text-primary);
   font: var(--nv-font-body);
   letter-spacing: var(--nv-letter-spacing-normal);