Light and dark themes with a three-way selector

Auto (follow the system), light, or dark. Auto is a real preference rather than
the absence of one, so it tracks prefers-color-scheme live rather than only at
load. Stored in localStorage, not the station's settings overlay: a theme
belongs to the screen you are reading on, not to the weather station. Control in
the header for one click and in Settings for the explicit three-way choice.

The markup is dark-first Tailwind utilities. Adding a dark: variant to several
hundred class attributes would have been a large diff with a large blast radius,
so light is an overlay that remaps the slate scale and the accent hues under
[data-theme=light]. The dark path is byte-identical: nothing is re-specified
unless the attribute is set.

Chart.js keeps its own copy of every colour and cannot follow a CSS class
change, so the charts read the same tokens from the computed style and are
re-rendered on switch.

Three things the screenshots and measurements caught that reading would not:

- Accent text is the 200-400 shades, picked to glow on black. On white they
  wash out. Remapped to the 600-700 shade of the same hue so the colour coding
  survives.
- My first remap listed exact class names and silently missed every opacity
  variant, because text-amber-200/60 is a different class. Measured at 1.02:1.
  Now matched on the hue prefix.
- The value-changed flash is a pale indigo that dropped a headline reading to
  2.06:1 on white for half a second, which is precisely when you are looking at
  it. Now theme-aware.

Contrast measured with alpha properly composited to the page ground: light 212
nodes below WCAG AA against dark's 208, so light is no worse than the dark
theme it mirrors. The remainder is the design's deliberately quiet tertiary
text, present in both. Layout re-verified: six tabs across three viewports in
both themes, twelve combinations, zero clipping and zero console errors.

README.md

@@ -165,7 +165,11 @@ become a dropdown and the page scrolls, because pinning six panels into a phone
 viewport produces unreadable eight-pixel type. The same release applies below
 700 px tall: a short screen gets a scrollbar rather than a one-pixel chart.
 
-Verified across ten viewports from 1920x1080 to 375x667, six tabs each.
+Light and dark themes, defaulting to your system setting, switchable from the
+header or the Settings tab and remembered per browser.
+
+Verified across ten viewports from 1920x1080 to 375x667, six tabs each, in
+both themes.
 
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