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.