[DeckSync] Address auto-sync review: disconnect, backfill and modal conflicts

- Fail the run when the connection drops while a sync is running, and clear
  the active conflict, so the queue is not sent on a dead client and a
  re-login as another account never receives this profile's uploads.
- Backfill last_modified from upload_time in the 37 -> 38 migration and drop
  the "last_modified is null" arm from the sync-check query, so rows that
  predate the column stop reporting as changed on every poll.
- Skip conflicts during auto runs and report how many were skipped, instead
  of opening the modal conflict dialog from a timer or a file-watcher event.

servatrice/src/serversocketinterface.cpp

@@ -904,10 +904,11 @@ bool AbstractServerSocketInterface::syncCheckHelper(int userId, qint64 sinceSecs
     // their files linger as tombstones, so walking the live folder tree would
     // miss deletes pointing at nonexistent folders. The client only checks
     // changed_files_size(), so folder paths are not reconstructed here.
+    // last_modified is backfilled by the 37 -> 38 migration, so a plain range
+    // check is enough.
     QSqlQuery *query = sqlInterface->prepareQuery("select id, guid, hash, last_modified, deleted from "
                                                   "{prefix}_decklist_files where id_user = :id_user "
-                                                  "and (last_modified is null or last_modified >= "
-                                                  "FROM_UNIXTIME(:since))");
+                                                  "and last_modified >= FROM_UNIXTIME(:since)");
     query->bindValue(":id_user", userId);
     query->bindValue(":since", sinceSecs);
     if (!sqlInterface->execSqlQuery(query)) {