fix: prevent hashtag preferences reset on empty or stale relay responses

This commit updates `HashtagPreferencesService.load` to handle transient empty responses from relays more robustly.
- It prevents `reset()` if `wasLoadedForUser` is true and the fetch returns empty, ensuring existing data isn't wiped during connectivity issues or relay switches.
- It adds a staleness check to ignore incoming events that are older than the currently loaded data, protecting against out-of-sync relays returning obsolete preferences.

This addresses the issue where hashtag interests would disappear ("turn gray") or revert to older states shortly after login when video grid filters were applied.

js/services/hashtagPreferencesService.js

@@ -430,9 +430,9 @@ class HashtagPreferencesService {
     if (!events.length) {
       // If we failed to fetch (network error) but already have data for this user,
       // preserve the existing state instead of wiping it.
-      if (fetchError && wasLoadedForUser) {
+      if (wasLoadedForUser) {
         userLogger.warn(
-          `${LOG_PREFIX} Keeping existing preferences after fetch error.`,
+          `${LOG_PREFIX} Keeping existing preferences despite empty relay response.`,
         );
         return;
       }
@@ -470,7 +470,7 @@ class HashtagPreferencesService {
     }, null);
 
     if (!latest) {
-      if (fetchError && wasLoadedForUser) {
+      if (wasLoadedForUser) {
         return;
       }
       this.reset();
@@ -478,6 +478,16 @@ class HashtagPreferencesService {
       return;
     }
 
+    const currentCreatedAt = Number(this.eventCreatedAt) || 0;
+    const latestCreatedAt = Number(latest.created_at) || 0;
+
+    if (wasLoadedForUser && currentCreatedAt > latestCreatedAt) {
+      userLogger.warn(
+        `${LOG_PREFIX} Ignoring stale preferences event (remote: ${latestCreatedAt}, local: ${currentCreatedAt}).`,
+      );
+      return;
+    }
+
     const decryptResult = await this.decryptEvent(latest, normalized);
     if (!decryptResult.ok) {
       userLogger.warn(