fix: add summary warning when all LAN sources are unreachable

When the network is down, sync_file() logs individual errors for each
file. Now run_once() detects when all files fail and emits a single
summary warning instead of proceeding with redundant "no changes" logs.

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

watch.py

@@ -22,6 +22,14 @@ def run_once(cfg):
     logger.info("开始同步 LAN 文件...")
     sync_results = sync_all_files(cfg, excel_dir)
 
+    # 如果全部同步失败，大概率是网络中断
+    all_failed = sync_results and all(s == 'error' for s, _ in sync_results.values())
+    if all_failed:
+        logger.warning(
+            f"所有 LAN 源文件不可达 ({len(sync_results)} 个)，可能网络中断，跳过本轮同步"
+        )
+        return
+
     # 2. Determine which tables need migration
     tables_to_migrate = determine_tables_to_migrate(cfg, sync_results)