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/Login.tsx

@@ -1,7 +1,7 @@
 export default function Login() {
   return (
     <div className="min-h-screen grid place-items-center bg-bg text-fg">
-      <div className="w-[min(92vw,420px)] rounded-2xl border border-border bg-card/70 backdrop-blur p-10 text-center shadow-2xl">
+      <div className="glass w-[min(92vw,420px)] rounded-2xl p-10 text-center">
         <div className="text-3xl font-bold tracking-tight">
           Sub<span className="text-accent">feed</span>
         </div>