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/lib/theme.ts

@@ -18,6 +18,8 @@ export interface ThemePrefs {
   fontScale: number;
   /** Show the per-scheme background image (dark theme only); off = flat colour. */
   bgImage: boolean;
+  /** How strongly the background image is faded out, 0–100% (higher = fainter). */
+  bgFade: number;
 }
 
 export const DEFAULT_THEME: ThemePrefs = {
@@ -25,6 +27,7 @@ export const DEFAULT_THEME: ThemePrefs = {
   scheme: "midnight",
   fontScale: 1.06,
   bgImage: true,
+  bgFade: 60,
 };
 
 export function applyTheme(t: ThemePrefs): void {
@@ -32,6 +35,7 @@ export function applyTheme(t: ThemePrefs): void {
   el.dataset.theme = t.mode;
   el.dataset.scheme = t.scheme;
   el.style.setProperty("--font-scale", String(t.fontScale));
+  if (typeof t.bgFade === "number") el.style.setProperty("--bg-fade", `${t.bgFade}%`);
 }
 
 export function loadLocalTheme(): ThemePrefs {