fix: persist session banner dismissals and add hide-banner setting

- Stop overwriting AI session titles with the "Planning session" fallback
  during cross-tab broadcasts before initialPlan has hydrated on resume.
- Persist SessionNotificationBanner dismissals to localStorage keyed by
  updatedAt so they survive refresh and auto-re-show on the next event.
- Add Settings → Appearance toggle to hide the banner entirely.
- Drop the inner scrollbar on the planning question list; let the outer
  pane handle all scrolling.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/hooks/useSessionBannerPref.ts

@@ -0,0 +1,45 @@
+import { useSyncExternalStore } from "react";
+
+const STORAGE_KEY = "fusion:hide-session-banners";
+const EVENT_NAME = "fusion:session-banner-pref-changed";
+
+function read(): boolean {
+  if (typeof window === "undefined") return false;
+  try {
+    return window.localStorage.getItem(STORAGE_KEY) === "1";
+  } catch {
+    return false;
+  }
+}
+
+function subscribe(onChange: () => void): () => void {
+  if (typeof window === "undefined") return () => {};
+  const handleStorage = (e: StorageEvent) => {
+    if (e.key === STORAGE_KEY) onChange();
+  };
+  const handleCustom = () => onChange();
+  window.addEventListener("storage", handleStorage);
+  window.addEventListener(EVENT_NAME, handleCustom);
+  return () => {
+    window.removeEventListener("storage", handleStorage);
+    window.removeEventListener(EVENT_NAME, handleCustom);
+  };
+}
+
+export function setSessionBannersHidden(hidden: boolean): void {
+  if (typeof window === "undefined") return;
+  try {
+    if (hidden) {
+      window.localStorage.setItem(STORAGE_KEY, "1");
+    } else {
+      window.localStorage.removeItem(STORAGE_KEY);
+    }
+    window.dispatchEvent(new Event(EVENT_NAME));
+  } catch {
+    // ignore
+  }
+}
+
+export function useSessionBannersHidden(): boolean {
+  return useSyncExternalStore(subscribe, read, () => false);
+}