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/Header.tsx

@@ -36,7 +36,7 @@ function ThemeMenu({
   setTheme: (t: ThemePrefs) => void;
 }) {
   return (
-    <div className="absolute right-0 mt-2 w-60 rounded-xl border border-border bg-surface shadow-2xl p-3 z-30">
+    <div className="glass absolute right-0 mt-2 w-60 rounded-xl p-3 z-30 animate-[popIn_0.16s_ease]">
       <div className="text-xs uppercase tracking-wide text-muted mb-2">Color scheme</div>
       <div className="grid grid-cols-4 gap-2 mb-3">
         {SCHEMES.map((s) => (
@@ -219,7 +219,7 @@ function AccountMenu({
       </button>
 
       {open && (
-        <div className="absolute right-0 mt-2 w-64 rounded-xl border border-border bg-surface shadow-2xl p-3 z-30">
+        <div className="glass absolute right-0 mt-2 w-64 rounded-xl p-3 z-30 animate-[popIn_0.16s_ease]">
           <div className="flex items-center gap-3 pb-3 border-b border-border">
             <Avatar me={me} className="w-10 h-10 text-sm shrink-0" />
             <div className="min-w-0">