fix: persist NIP-5C scroll parameter values and encoding settings

Store scroll settings (param values, endianness, presence bytes) in
localStorage keyed by event ID. Loads persisted values on mount, saves
on change (skipping initial mount to avoid unnecessary writes). Filters
stale param keys when scroll parameters change between versions.

Closes #266

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/nostr/kinds/ScrollRenderer.tsx

@@ -148,7 +148,11 @@ export function ScrollDetailRenderer({ event }: { event: NostrEvent }) {
         )}
       </div>
 
-      <ScrollExecutor params={params} wasmBase64={event.content} />
+      <ScrollExecutor
+        params={params}
+        wasmBase64={event.content}
+        eventId={event.id}
+      />
     </div>
   );
 }

src/components/scroll/ScrollExecutor.tsx

@@ -21,9 +21,48 @@ interface ScrollExecutorProps {
   params: ScrollParam[];
   /** Base64-encoded WASM binary */
   wasmBase64: string;
+  /** Event ID used as localStorage key for persisting settings */
+  eventId?: string;
 }
 
-export function ScrollExecutor({ params, wasmBase64 }: ScrollExecutorProps) {
+const SCROLL_STORAGE_PREFIX = "scroll_settings_";
+
+function loadScrollSettings(eventId: string): {
+  paramValues?: Record<string, string>;
+  endianness?: "LE" | "BE";
+  presenceBytes?: boolean;
+} {
+  try {
+    const stored = localStorage.getItem(SCROLL_STORAGE_PREFIX + eventId);
+    return stored ? JSON.parse(stored) : {};
+  } catch {
+    return {};
+  }
+}
+
+function saveScrollSettings(
+  eventId: string,
+  settings: {
+    paramValues: Record<string, string>;
+    endianness: "LE" | "BE";
+    presenceBytes: boolean;
+  },
+) {
+  try {
+    localStorage.setItem(
+      SCROLL_STORAGE_PREFIX + eventId,
+      JSON.stringify(settings),
+    );
+  } catch {
+    // localStorage full or unavailable — silently ignore
+  }
+}
+
+export function ScrollExecutor({
+  params,
+  wasmBase64,
+  eventId,
+}: ScrollExecutorProps) {
   const { pubkey } = useAccount();
   const { relays: relayStates } = useRelayState();
 
@@ -31,17 +70,28 @@ export function ScrollExecutor({ params, wasmBase64 }: ScrollExecutorProps) {
     .filter(([, state]) => state.connectionState === "connected")
     .map(([url]) => url);
 
-  // Pre-fill "me" params with logged-in pubkey
+  // Load persisted settings
+  const stored = eventId ? loadScrollSettings(eventId) : {};
+
+  // Pre-fill "me" params with logged-in pubkey, then overlay persisted values
   const defaultValues: Record<string, string> = {};
   for (const p of params) {
     if (p.name === "me" && p.type === "public_key" && pubkey) {
       defaultValues[p.name] = pubkey;
     }
   }
+  // Filter stored values to only include current params (remove stale keys)
+  const validParamNames = new Set(params.map((p) => p.name));
+  const filteredStored = Object.fromEntries(
+    Object.entries(stored.paramValues || {}).filter(([k]) =>
+      validParamNames.has(k),
+    ),
+  );
+  const initialValues = { ...defaultValues, ...filteredStored };
 
   const [runtimeState, setRuntimeState] = useState<ScrollRuntimeState>("idle");
   const [paramValues, setParamValues] =
-    useState<Record<string, string>>(defaultValues);
+    useState<Record<string, string>>(initialValues);
   const [displayedEventsMap, setDisplayedEventsMap] = useState<
     Map<string, NostrEvent>
   >(new Map());
@@ -50,10 +100,15 @@ export function ScrollExecutor({ params, wasmBase64 }: ScrollExecutorProps) {
   const [activeSubs, setActiveSubs] = useState<SubscriptionInfo[]>([]);
   const [eventCount, setEventCount] = useState(0);
   const controllerRef = useRef<ScrollRuntimeController | null>(null);
+  const isInitialMount = useRef(true);
 
   // Encoding options
-  const [endianness, setEndianness] = useState<"LE" | "BE">("BE");
-  const [presenceBytes, setPresenceBytes] = useState(false);
+  const [endianness, setEndianness] = useState<"LE" | "BE">(
+    stored.endianness || "BE",
+  );
+  const [presenceBytes, setPresenceBytes] = useState(
+    stored.presenceBytes ?? false,
+  );
 
   const isActive = runtimeState === "loading" || runtimeState === "running";
 
@@ -70,6 +125,16 @@ export function ScrollExecutor({ params, wasmBase64 }: ScrollExecutorProps) {
     (p) => p.required && !paramValues[p.name]?.trim(),
   );
 
+  // Persist settings to localStorage when they change (skip initial mount)
+  useEffect(() => {
+    if (!eventId) return;
+    if (isInitialMount.current) {
+      isInitialMount.current = false;
+      return;
+    }
+    saveScrollSettings(eventId, { paramValues, endianness, presenceBytes });
+  }, [eventId, paramValues, endianness, presenceBytes]);
+
   useEffect(() => {
     return () => {
       controllerRef.current?.stop();