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/lib/hints.ts

@@ -0,0 +1,36 @@
+// App-wide "hints" toggle: when on, Tooltip components reveal short explanatory
+// captions on hover so the UI is somewhat self-documenting for new users. Experienced
+// users can turn it off in Settings. Persisted to localStorage (+ server preferences).
+const KEY = "subfeed.hints";
+
+let enabled = load();
+let listeners: Array<() => void> = [];
+
+function load(): boolean {
+  try {
+    return localStorage.getItem(KEY) !== "0"; // default ON
+  } catch {
+    return true;
+  }
+}
+
+export function hintsEnabled(): boolean {
+  return enabled;
+}
+
+export function setHintsEnabled(v: boolean): void {
+  enabled = v;
+  try {
+    localStorage.setItem(KEY, v ? "1" : "0");
+  } catch {
+    /* ignore */
+  }
+  listeners.forEach((l) => l());
+}
+
+export function subscribeHints(listener: () => void): () => void {
+  listeners.push(listener);
+  return () => {
+    listeners = listeners.filter((l) => l !== listener);
+  };
+}