fix(a11y): give the semantic colours a ramp, and every fill a foreground

The contrast pass found two things larger than itself, both recorded as
not-fixed. This is them.

The semantic colours were 'fixed across themes', and one fixed colour
cannot clear 4.5:1 against both a near-black and a near-white surface:
--yj-error measured 2.55:1 on dark's elevated, --yj-info 2.31:1, and
success and warning failed on dark and light both. They are split by the
question they answer. A *fill* is 'what colour is a danger button' --
red in every theme, unchanged -- and a *text* colour is 'what colour is
the word failed on this background', which is now per ramp.

Every fill also carries a computed foreground. White on the default
accent is 1.43:1, and the accent is a colour picker, so no fixed answer
survives it: --yj-accent-fg and the four semantic -fg values are derived
(white if white clears, else black), which keeps a red danger button
white and flips a green or amber one to black. Two accent buttons took
their foreground from --yj-bg-base, which inverts with the ramp -- that
is exactly the white-on-yellow 'Apply (A)' the light theme showed.

Accent used as text gets the same treatment through accentTextOn(),
which mixes along the hue until it clears the ramp's surface and stops.
On both dark ramps it returns the accent unchanged, so the dark themes
are visually untouched by that half.

Measured across three ramps and twelve views: 2237 nodes, 0 failing,
against 110 on dark and 50 on light before. Borders, outlines and
shadows were explicitly kept on the fill token -- a border is not text,
and the first pass of the rewrite moved 30 of them by accident.

frontend/test/components/theme-contrast.test.ts

@@ -41,7 +41,23 @@ function contrast(a: string, b: string): number {
   return (hi + 0.05) / (lo + 0.05);
 }
 
-const TEXT = ['textPrimary', 'textSecondary', 'textTertiary'] as const;
+const TEXT = [
+  'textPrimary',
+  'textSecondary',
+  'textTertiary',
+  'successText',
+  'warningText',
+  'errorText',
+  'infoText',
+] as const;
+
+/**
+ * The semantic *fills*, which are fixed across ramps because a danger
+ * button is red in every theme. What varies is the foreground, and it
+ * is computed rather than written down because the accent is a colour
+ * picker: white on the default `#ffd43b` is 1.43:1.
+ */
+const FILLS = ['#2f9e44', '#e8590c', '#e03131', '#4263eb', '#ffd43b'];
 
 /**
  * `bgOverlay` is deliberately absent for tertiary on the dark ramp.
@@ -54,8 +70,17 @@ const SURFACES: Record<(typeof TEXT)[number], (keyof ShadePalette)[]> = {
   textPrimary: ['bgBase', 'bgSurface', 'bgElevated', 'bgOverlay'],
   textSecondary: ['bgBase', 'bgSurface', 'bgElevated'],
   textTertiary: ['bgBase', 'bgSurface', 'bgElevated'],
+  successText: ['bgBase', 'bgSurface', 'bgElevated'],
+  warningText: ['bgBase', 'bgSurface', 'bgElevated'],
+  errorText: ['bgBase', 'bgSurface', 'bgElevated'],
+  infoText: ['bgBase', 'bgSurface', 'bgElevated'],
 };
 
+/** Black or white, whichever reads on a fill — `readableOn`'s rule. */
+function readable(fill: string): string {
+  return contrast(fill, '#ffffff') >= 4.5 ? '#ffffff' : '#000000';
+}
+
 describe('theme contrast', () => {
   const cases = Object.entries(SHADE_PALETTES).flatMap(([shade, palette]) =>
     TEXT.flatMap((text) =>
@@ -72,13 +97,32 @@ describe('theme contrast', () => {
     expect(ratio).toBeGreaterThanOrEqual(4.5);
   });
 
+  // Every fill in the app has a foreground that reads on it, for any
+  // accent a user can pick — which is the half a fixed `color: #000`
+  // got right only for the current default, and `var(--yj-bg-base)`
+  // got backwards on the light ramp (white on yellow, 1.43:1).
+  it.each(FILLS)('%s carries a readable foreground', (fill) => {
+    expect(contrast(fill, readable(fill))).toBeGreaterThanOrEqual(4.5);
+  });
+
   // Sizing tertiary to clear 4.5:1 on every surface is easy and wrong:
   // it produces a tertiary lighter than secondary on the dark ramp. The
   // ramp has to stay a ramp, or "tertiary" stops meaning anything.
   it.each(Object.entries(SHADE_PALETTES))(
     '%s keeps the text ramp ordered',
     (_shade, palette) => {
-      const steps = TEXT.map((t) => contrast(palette[t], palette.bgSurface));
+      // The greyscale ramp only. The semantic text colours are not a
+      // ramp — they are four hues that each have to clear the same bar,
+      // and ordering them against each other means nothing.
+      const greyscale = [
+        'textPrimary',
+        'textSecondary',
+        'textTertiary',
+      ] as const;
+
+      const steps = greyscale.map((t) =>
+        contrast(palette[t], palette.bgSurface),
+      );
 
       expect(steps).toEqual([...steps].sort((a, b) => b - a));
     },