fix(a11y): make every text colour clear WCAG AA on every ramp

a11y.md flagged --yj-text-tertiary on --yj-bg-surface as 'borderline
(~4.1:1) but that needs a real measurement', and plan 007 parked it as
'worth measuring before planning'. Measured, against the rendered app
and then across all three background ramps: it failed AA in nine of
twelve text/surface combinations, as low as 2.31:1 on dark's overlay
and 2.55:1 on light's -- the app's most-used secondary text colour,
failing on every view. Not borderline. 110 failing nodes across twelve
views, now 0 of 659.

Three separate mechanisms, and only the first is the finding:

- The ramps. Tertiary is raised per ramp (#a6a6a6 dark, #949494 darker,
  #5c636a light), sized to the lightest surface it actually sits on and
  keeping its hue. Sizing it to bgOverlay too would need a grey lighter
  than secondary, so bgOverlay is documented as not a text surface and
  the one component that put text there uses primary.
- The avatar generator. hsl(hue, 45%, 35%) behind white initials failed
  for 35 of the 360 hues -- the yellow-green band -- so which artists
  were unreadable depended on how their names hashed. The two a sweep
  found were not the finding. 32% clears every hue.
- Jobs' local #ff6b6b, at 4.15:1 on elevated.

Pinned by a unit test over the palette table rather than a DOM sweep:
the ramps are pure data, and checking only what happens to be on screen
is exactly how the light ramp went unexamined. Note that make ui-visual
cannot see any of this -- the component tier renders the fallbacks,
because theme-store sets :root only in the real app.

frontend/src/components/explore-view/explore-view.ts

@@ -1,3 +1,4 @@
+import { avatarBackground } from '@utils/avatar-color';
 import { LitElement, html, css, nothing } from 'lit';
 import { customElement, state, query as litQuery } from 'lit/decorators.js';
 import '@components/page-header/page-header';
@@ -61,15 +62,6 @@ const MAX_SECTION_RESULTS = 10;
 const THUMBNAIL_CACHE_LIMIT = 96;
 
 
-/** Hash a string to a hue value 0–360 for avatar coloring. */
-function nameToHue(name: string): number {
-    let hash = 0;
-    for (let i = 0; i < name.length; i++) {
-        hash = name.charCodeAt(i) + ((hash << 5) - hash);
-    }
-    return Math.abs(hash) % 360;
-}
-
 /** Format milliseconds as mm:ss. */
 function formatDuration(ms: number): string {
     if (!ms || ms <= 0) return '';
@@ -1734,7 +1726,6 @@ export class ExploreView extends ViewLifecycleMixin(LitElement) {
                     : nothing}
                 <div class="horizontal-row">
                     ${artists.map((a) => {
-                        const hue = nameToHue(a.name);
                         return html`
                             <div
                                 class="artist-card"
@@ -1750,7 +1741,7 @@ export class ExploreView extends ViewLifecycleMixin(LitElement) {
                             >
                                 <div
                                     class="artist-avatar"
-                                    style="background: hsl(${hue}, 45%, 35%)"
+                                    style="background: ${avatarBackground(a.name)}"
                                 >
                                     ${this.artistImageCache.get(a.mbid)
                                         ? html`<img