fix: preserve ENS names on lookup failure, add debug logging (closes #22)

Two issues that could cause ENS names to disappear:

1. refreshBalances: on ENS lookup error, addr.ensName was set to null,
   wiping any previously resolved name. Now keeps the existing value
   on error — only overwrites on successful lookup.

2. ens.js cache: failed lookups were cached as null for 12 hours,
   preventing retries even after transient errors resolved. Now skips
   caching on failure so subsequent lookups retry immediately.

Added debug logging to ENS reverse lookups in refreshBalances.

src/shared/balances.js

@@ -123,15 +123,27 @@ async function refreshBalances(wallets, rpcUrl, blockscoutUrl, trackedTokens) {
                     }),
             );
 
-            // ENS reverse lookup
+            // ENS reverse lookup — only overwrite on success so that
+            // transient RPC errors don't wipe a previously resolved name.
             updates.push(
                 provider
                     .lookupAddress(addr.address)
                     .then((name) => {
                         addr.ensName = name || null;
+                        log.debugf(
+                            "ENS reverse",
+                            addr.address,
+                            "->",
+                            addr.ensName,
+                        );
                     })
-                    .catch(() => {
-                        addr.ensName = null;
+                    .catch((e) => {
+                        log.errorf(
+                            "ENS reverse failed",
+                            addr.address,
+                            e.message,
+                        );
+                        // Keep existing addr.ensName if we had one
                     }),
             );
 

src/shared/ens.js

@@ -39,7 +39,7 @@ async function resolveEnsName(address, rpcUrl) {
         return name;
     } catch (e) {
         log.errorf("ENS reverse lookup failed", address, e.message);
-        setCache(address, null);
+        // Don't cache failures — let subsequent lookups retry
         return null;
     }
 }