Rework light theme, add format wheel and motion, fix Firefox paths

Theme: light is a dimmer brand-tinted stone with deeper shade, so it reads
as the same material as dark; every neutral now carries --nv-brand in both
appearances. Native controls (accent-color, caret-color) take the brand.

Time picker: 24-hour by default; data-nv-format-toggle adds a third wheel
of 24H / AM / PM. Swiping it rebuilds the hour wheel for the new format.
The selection lens is a raised glass bar, not a flat band.

Motion: drum effect on the wheels as a scroll-driven animation, with a
script fallback for engines without it (Firefox); rubber-band at both ends
of a wheel that springs back; calendar months slide locally instead of via
a page-wide transition; a chosen day springs into place.

Firefox: wheel notches arrive in lines, not pixels, which made the
rubber-band invisible; deltaMode is now normalised. The selected-day pop
no longer plays on page load.

Also: hover only inside @media (hover: hover) on the slider, the Glaze
inner glow is a token, and the focus note in GUIDELINES.md is corrected
(popovers do not trap focus). SPDX identifier added to niveus.js.

check.py, build.py and build.py --check pass.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

niveus/tokens/materials.css

@@ -54,6 +54,9 @@
   --nv-glaze-tint-min: 4%;     /* tint opacity at frost 0 */
   --nv-glaze-tint-max: 78%;    /* tint opacity at frost 1 */
   --nv-glaze-rim: 1.5px;
+  /* The light a pane holds just inside its edge, so it reads as a thick lens
+     rather than a flat film. */
+  --nv-glaze-glow: inset 0 0 1.5rem -0.5rem var(--nv-color-glaze-specular);
 
   /* --- Sheen ------------------------------------------------------------------
      raised    neutral controls: buttons, the selected tab, a titlebar