feat(appearance): opt-in glass tuner toggle in Settings

Graduate the floating glass tuner from a localhost-only dev tool to a
user-facing opt-in: it's now gated on a per-user `theme.showTuner` pref (off by
default) with a "Show the glass tuner" switch in Settings → Appearance, riding
the existing theme draft-save. Softened the dev-only framing. Behaviour and look
are unchanged; the full preset system is a separate epic.

frontend/src/lib/theme.ts

@@ -20,6 +20,8 @@ export interface ThemePrefs {
   bgImage: boolean;
   /** How strongly the background image is faded out, 0–100% (higher = fainter). */
   bgFade: number;
+  /** Show the floating glass/theme tuner (a secondary "geek toy"); off by default. */
+  showTuner: boolean;
 }
 
 export const DEFAULT_THEME: ThemePrefs = {
@@ -28,6 +30,7 @@ export const DEFAULT_THEME: ThemePrefs = {
   fontScale: 1.06,
   bgImage: true,
   bgFade: 60,
+  showTuner: false,
 };
 
 export function applyTheme(t: ThemePrefs): void {