feat(appearance): opt-in glass tuner toggle in Settings

Graduate the floating glass tuner from a localhost-only dev tool to a
user-facing opt-in: it's now gated on a per-user `theme.showTuner` pref (off by
default) with a "Show the glass tuner" switch in Settings → Appearance, riding
the existing theme draft-save. Softened the dev-only framing. Behaviour and look
are unchanged; the full preset system is a separate epic.

frontend/src/components/SettingsPanel.tsx

@@ -205,6 +205,16 @@ 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.textSize")}>