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/App.tsx

@@ -382,13 +382,13 @@ export default function App() {
     setAccountRaw(PERF_KEY, perf ? "1" : "0");
   }, [perf]);
 
-  // The per-scheme background image (the "glass over image" look) is on only in dark theme, when
-  // the user hasn't opted out, and not in perf mode. Drives `html[data-backdrop]` (see index.css),
-  // which both paints the image on <body> and switches the glass to its translucent tier.
+  // The per-scheme background image (the "glass over image" look) is on when the user hasn't opted
+  // out and we're not in perf mode. Drives `html[data-backdrop]` (see index.css), which paints the
+  // theme-appropriate image on <body> (dark or /light/ set) and switches the glass to translucent.
   useEffect(() => {
-    const on = theme.mode === "dark" && theme.bgImage && !perf;
+    const on = theme.bgImage && !perf;
     document.documentElement.dataset.backdrop = on ? "on" : "off";
-  }, [theme.mode, theme.bgImage, perf]);
+  }, [theme.bgImage, perf]);
   useEffect(() => setHintsEnabled(hints), [hints]);
   useEffect(() => configureNotifications(notif), [notif]);