Increase receive buffer from 8KB to 256KB in synchronous_query/publish functions

The 8KB buffer was too small for large Nostr events (e.g. long-form
content in kind 30023). The pool's query_sync already uses 256KB.
This matches that size to avoid truncation when using the standalone
synchronous query functions for backfill.

nostr_core/core_relays.c

@@ -196,7 +196,7 @@ cJSON** synchronous_query_relays_with_progress(
             }
             
             // Try to receive message (short timeout to keep UI responsive)
-            char buffer[8192];
+            char buffer[262144];
             int len = nostr_ws_receive(relay->client, buffer, sizeof(buffer)-1, 50);
             
             if (len > 0) {
@@ -580,7 +580,7 @@ publish_result_t* synchronous_publish_event_with_progress(
             }
             
             // Try to receive message (short timeout to keep UI responsive)
-            char buffer[8192];
+            char buffer[262144];
             int len = nostr_ws_receive(relay->client, buffer, sizeof(buffer)-1, 50);
             
             if (len > 0) {