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/base.css

@@ -134,6 +134,14 @@ textarea {
   border-radius: var(--nv-radius-xs);
 }
 
+/* Whatever the browser draws itself — a bare checkbox, a progress bar, the
+   text caret, a date field's glyphs — takes the brand, so no native colour
+   survives next to the system's own. */
+:root {
+  accent-color: var(--nv-color-accent);
+  caret-color: var(--nv-color-accent);
+}
+
 ::selection {
   background-color: var(--nv-color-selection);
   color: var(--nv-color-text-primary);