feat(design): the scheme set, the colour contract and the token utilities (E28 S3a)

Four schemes (Midnight; Forest cyan, Slate fuchsia, Matrix lime), YouTube
and Starship retired with no data migration: normalizeTheme maps a stored
retired scheme to Midnight. A vitest contract measures every scheme x mode
(text 4.5:1, control edge 3:1, status text, the accent's 30-degree hue gap);
it caught light-mode danger at 4.43:1, now red-700. The state layers, the
strong edge, the touch target and the fades are index.css tokens and
utilities the lab now reads. Backdrops recoloured to the new accents, with
a grey overlay grain that actually breaks the banding (longest flat run
15-26px -> 4-8px). The glass tuner is retired.

frontend/src/components/SettingsPanel.tsx

@@ -213,16 +213,6 @@ function Appearance({ prefs }: { prefs: PrefsController }) {
         >
           <Switch label={t("settings.appearance.showHints")} checked={hints} onChange={setHints} />
         </SettingRow>
-        <SettingRow
-          label={t("settings.appearance.showTuner")}
-          hint={t("settings.appearance.showTunerHint")}
-        >
-          <Switch
-            label={t("settings.appearance.showTuner")}
-            checked={theme.showTuner}
-            onChange={(v) => setTheme({ ...theme, showTuner: v })}
-          />
-        </SettingRow>
       </Section>
 
       <Section title={t("settings.appearance.startup")}>