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);
+  };
+}

frontend/src/lib/notifications.ts

@@ -83,6 +83,7 @@ function beep(): void {
   try {
     const Ctx = window.AudioContext || (window as unknown as { webkitAudioContext: typeof AudioContext }).webkitAudioContext;
     audioCtx = audioCtx || new Ctx();
+    if (audioCtx.state === "suspended") void audioCtx.resume();
     const osc = audioCtx.createOscillator();
     const gain = audioCtx.createGain();
     osc.connect(gain);
@@ -151,11 +152,14 @@ export function notify(input: NotifyInput): number {
   const id = counter++;
   const requiresInteraction = input.requiresInteraction ?? false;
   const level = input.level ?? "info";
+  // config.durationMs === 0 means "don't auto-dismiss" (toast stays until dismissed).
   const duration = requiresInteraction
     ? undefined
     : level === "error" || level === "fatal"
       ? ERROR_TTL
-      : config.durationMs;
+      : config.durationMs > 0
+        ? config.durationMs
+        : undefined;
   items = [
     ...items,
     {