feat(glass): user-facing Image-fade slider under Settings > Background image

Replaces the dev-only backdrop-fade knob with a real control: when Background
image is on (dark theme), a 0-100% 'Image fade' slider appears beneath it and
drives --bg-fade live (higher = fainter). Persists to DB + localStorage via the
existing theme prefs path (ThemePrefs.bgFade, applyTheme sets the CSS var).
Removed --bg-fade from the GlassTuner so the two don't fight over the var.

frontend/src/components/GlassTuner.tsx

@@ -38,7 +38,6 @@ const SLIDERS: Slider[] = [
   { k: "--glass-inset", label: "Top highlight", min: 0, max: 40, step: 1, unit: "%", def: 15 },
   { k: "--glass-scrim", label: "Under-text scrim", min: 0, max: 85, step: 1, unit: "%", def: 30 },
   { k: "--ambient", label: "Ambient intensity", min: 0, max: 2, step: 0.05, unit: "", def: 1.1 },
-  { k: "--bg-fade", label: "Backdrop fade", min: 40, max: 97, step: 1, unit: "%", def: 60 },
 ];
 
 const PALETTE: { k: string; label: string }[] = [