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/smart-playlist-editor/smart-playlist-editor.ts

@@ -287,7 +287,7 @@ export class SmartPlaylistEditor extends LitElement {
             }
 
             .remove-btn:hover {
-                color: #ff6b6b;
+                color: #ff8787;
                 background: rgba(255, 107, 107, 0.1);
             }
 
@@ -411,7 +411,7 @@ export class SmartPlaylistEditor extends LitElement {
 
             .preview-error {
                 font-size: var(--yj-text-sm);
-                color: #ff6b6b;
+                color: #ff8787;
                 padding: 6px 0;
             }