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/VideoCard.tsx

@@ -170,7 +170,7 @@ export default function VideoCard({
   return (
     <div
       className={clsx(
-        "group rounded-2xl border border-border bg-card/50 p-2.5 shadow-sm transition-all duration-150 hover:-translate-y-1 hover:shadow-2xl hover:bg-card hover:border-accent/40",
+        "group glass-card glass-hover rounded-2xl p-2.5 transition-all duration-150 hover:-translate-y-1",
         watched && "opacity-55"
       )}
     >