fix: don't let a transient fetch failure wipe local chains/assets

ChainSyncService.syncUp() and EvmAssetsSyncService.syncEVMAssets() both
diff a freshly-fetched remote list against everything currently stored
locally, then apply that diff unconditionally. If the remote fetch
succeeds but returns a suspiciously small or empty list (CDN hiccup,
regional network filtering, a bad publish upstream) - rather than
throwing, which retryUntilDone would catch and retry - the diff would
delete most or all of the user's chains/assets from the local DB. This
is active data loss, not a sync failure, and it self-heals on the next
good sync if we just skip applying a suspicious one instead.

Directly relevant: a user's live production install (unrelated to any
in-flight branch work) was observed with a completely empty networks
and token list after previously working fine, matching this exact
failure mode.

runtime/src/main/java/io/novafoundation/nova/runtime/multiNetwork/asset/EvmAssetsSyncService.kt

@@ -1,7 +1,9 @@
 package io.novafoundation.nova.runtime.multiNetwork.asset
 
+import android.util.Log
 import com.google.gson.Gson
 import io.novafoundation.nova.common.utils.CollectionDiffer
+import io.novafoundation.nova.common.utils.LOG_TAG
 import io.novafoundation.nova.common.utils.retryUntilDone
 import io.novafoundation.nova.core_db.dao.ChainAssetDao
 import io.novafoundation.nova.core_db.dao.ChainDao
@@ -40,6 +42,18 @@ class EvmAssetsSyncService(
                 new.copy(enabled = old?.enabled ?: ENABLED_DEFAULT_BOOL)
             }
 
+        // Same defensive guard as ChainSyncService: a transient upstream issue can make the fetch return
+        // successfully with a suspiciously small/empty list. Diffing that against a populated local DB would
+        // delete most or all of the user's ERC20 tokens (e.g. USDT-ERC20) - skip instead of wiping good data.
+        if (oldAssets.isNotEmpty() && newAssets.size < oldAssets.size / 2) {
+            Log.e(
+                LOG_TAG,
+                "Refusing to apply EVM asset sync: remote returned ${newAssets.size} assets vs ${oldAssets.size} currently stored " +
+                    "(would remove more than half). Likely a transient fetch issue - skipping this sync cycle."
+            )
+            return
+        }
+
         val diff = CollectionDiffer.findDiff(newAssets, oldAssets, forceUseNewItems = false)
         chainAssetDao.updateAssets(diff)
     }

runtime/src/main/java/io/novafoundation/nova/runtime/multiNetwork/chain/ChainSyncService.kt

@@ -1,7 +1,9 @@
 package io.novafoundation.nova.runtime.multiNetwork.chain
 
+import android.util.Log
 import com.google.gson.Gson
 import io.novafoundation.nova.common.utils.CollectionDiffer
+import io.novafoundation.nova.common.utils.LOG_TAG
 import io.novafoundation.nova.common.utils.retryUntilDone
 import io.novafoundation.nova.core_db.dao.ChainDao
 import io.novafoundation.nova.core_db.dao.FullAssetIdLocal
@@ -40,6 +42,21 @@ class ChainSyncService(
 
         val remoteChains = retryUntilDone { chainFetcher.getChains() }
 
+        // A transient upstream issue (CDN hiccup, regional network filtering, a bad publish) can make
+        // chainFetcher.getChains() return successfully with a suspiciously small/empty list instead of
+        // throwing. Applying that as a diff against a populated local DB would delete most or all of the
+        // user's chains/assets - not a sync failure, but active data loss, for something that self-heals on
+        // the next successful sync if we just skip applying it. Only guard when we HAD data: an empty result
+        // on a genuinely first-ever sync is normal and must proceed.
+        if (oldChains.isNotEmpty() && remoteChains.size < oldChains.size / 2) {
+            Log.e(
+                LOG_TAG,
+                "Refusing to apply chain sync: remote returned ${remoteChains.size} chains vs ${oldChains.size} currently stored " +
+                    "(would remove more than half). Likely a transient fetch issue - skipping this sync cycle."
+            )
+            return@withContext
+        }
+
         val newChains = remoteChains.map { mapRemoteChainToLocal(it, oldChainsById[it.chainId], source = ChainLocal.Source.DEFAULT, gson) }
         val newAssets = remoteChains.flatMap { chain ->
             chain.assets.map {