fix(datastore): prefer named default entry when two default dirs collide in ListAllDevices

When the same device appears under `default/` in two separate data dirs
(e.g. primary DataDir and the legacy st-go/data path), the first-seen entry
was kept unconditionally even when it had an empty name. A subsequent
default entry carrying a real name was silently dropped, causing name loss
in SyncFromAccountFull.

Addresses TestReproduceMissingName regression introduced by the
dedup-default-last change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/service/datastore/datastore.go

@@ -681,7 +681,14 @@ func (ds *DataStore) ListAllDevices() ([]models.ServiceDeviceInfo, error) {
 				}
 
 				// "default" never replaces a real-account entry.
+				// But when two "default" entries collide across data dirs,
+				// prefer the one with a non-empty name (more information).
 				if info.AccountID == accountIDDefault {
+					if entry.account == accountIDDefault && devices[entry.index].Name == "" && info.Name != "" {
+						devices[entry.index] = info
+						seenIDs[key] = seenEntry{index: entry.index, account: info.AccountID}
+					}
+
 					continue
 				}