feat(ui): liquid-glass design system, settings polish, hints, notif fixes

- Add a theme-aware glass surface system (.glass/.glass-card + ambient backdrop,
  performance-mode opt-out) and apply it across panels, popovers, toasts, cards,
  sidebar widgets, channel rows, video cards and login.
- SettingsPanel: slide in/out animation, glass styling, wrapping pill tabs (no
  horizontal scrollbar) with a prominent active state.
- Notifications: auto-dismiss can be switched off (stays until closed); the test
  notification now also triggers the alert sound; resume a suspended AudioContext.
- Add an app-wide, toggleable hint/tooltip system (lib/hints + Tooltip) and wire
  hints across the settings and channel-manager surfaces; persisted per account.

frontend/src/components/Toaster.tsx

@@ -35,7 +35,7 @@ export default function Toaster() {
         return (
           <div
             key={t.id}
-            className="relative overflow-hidden bg-surface border border-border rounded-xl shadow-2xl px-3 py-3 flex items-start gap-3"
+            className="glass relative overflow-hidden rounded-xl px-3 py-3 flex items-start gap-3 animate-[popIn_0.16s_ease]"
           >
             <Icon className={`w-5 h-5 shrink-0 mt-0.5 ${color}`} />
             <div className="min-w-0 flex-1">