feat(glass): light-theme background images + enable the toggle in light

Add a light set of the 6 per-scheme backdrops (public/backdrops/light/*.svg) —
soft, pastel-mid, non-figurative washes tuned to each accent so they read through
the light translucent glass. Wire them up centrally:
- App: the backdrop is on whenever bgImage && !perf (was dark-only), so light
  gets it too.
- index.css: per-scheme rules are now [data-theme]-qualified — dark uses the dark
  set, light the /light/ set; shared cover/fixed size rule for both.
- Settings: the Background image toggle + Image-fade slider work in light now
  (removed the dark-only gates); hint no longer says 'dark only'. Trilingual.
The light glass (62% translucent tier) refracts the light backdrop just like dark
refracts its own; one fade slider controls both.

frontend/src/components/SettingsPanel.tsx

@@ -165,11 +165,10 @@ function Appearance({ prefs }: { prefs: PrefsController }) {
           <Switch
             label={t("settings.appearance.backgroundImage")}
             checked={theme.bgImage}
-            disabled={theme.mode !== "dark"}
             onChange={(v) => setTheme({ ...theme, bgImage: v })}
           />
         </SettingRow>
-        {theme.mode === "dark" && theme.bgImage && (
+        {theme.bgImage && (
           <div className="pl-1 pb-1">
             <div className="flex items-center justify-between text-xs text-muted mb-1">
               <span>{t("settings.appearance.backgroundFade")}</span>