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/Sidebar.tsx

@@ -401,7 +401,7 @@ export default function Sidebar({
       )}
 
       {filters.channelId && !editing && (
-        <div className="rounded-xl border border-border bg-card/30">
+        <div className="glass-card rounded-xl">
           <div className="px-3 pt-2 text-xs uppercase tracking-wide text-muted">Channel</div>
           <div className="px-3 pb-3 pt-2">
             <button
@@ -474,7 +474,7 @@ function WidgetCard({
     <div
       ref={setNodeRef}
       style={style}
-      className={`rounded-xl border border-border bg-card/30 ${
+      className={`glass-card rounded-xl ${
         isDragging ? "relative z-10 shadow-2xl ring-1 ring-accent" : ""
       } ${hidden && editing ? "opacity-50" : ""}`}
     >