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/App.tsx

@@ -16,6 +16,7 @@ import {
   type SidebarLayout,
 } from "./lib/sidebarLayout";
 import { configureNotifications } from "./lib/notifications";
+import { setHintsEnabled } from "./lib/hints";
 import Login from "./components/Login";
 import Header from "./components/Header";
 import Sidebar from "./components/Sidebar";
@@ -100,6 +101,9 @@ export default function App() {
       saveLayoutLocal(l);
     }
     if (prefs.notifications) configureNotifications(prefs.notifications);
+    if (typeof prefs.hints === "boolean") setHintsEnabled(prefs.hints);
+    if (typeof prefs.performanceMode === "boolean")
+      document.documentElement.dataset.perf = prefs.performanceMode ? "1" : "";
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [meQuery.data?.id]);