[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.

cockatrice/src/interface/deck_sync/deck_sync_manager.h

@@ -159,6 +159,7 @@ private:
         qint64 lastModifiedSecs = 0;
     };
 
+    void startSync(bool autoRun);
     void failSync(const QString &error);
     void finishSync();
     [[nodiscard]] SyncSummary currentSummary() const;
@@ -240,6 +241,8 @@ private:
     int downloadedCount = 0;
     int unchangedCount = 0;
     int skippedCount = 0;
+    int conflictCount = 0;
+    bool autoRun = false;
 };
 
 #endif // DECK_SYNC_MANAGER_H