Backfill missing default keys into sequences on JSON load

Ensures all sequences have required keys like mode, cfg, etc.
from DEFAULTS via setdefault so they persist on next save.

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

utils.py

@@ -144,6 +144,15 @@ def save_snippets(snippets):
         json.dump(snippets, f, indent=4)
     os.replace(tmp, SNIPPETS_FILE)
 
+def _ensure_default_keys(data: dict) -> None:
+    """Ensure all sequences have required keys from DEFAULTS."""
+    for item in data.get(KEY_BATCH_DATA, []):
+        if not isinstance(item, dict):
+            continue
+        for k, v in DEFAULTS.items():
+            item.setdefault(k, v)
+
+
 def _migrate_lora_keys(data: dict) -> None:
     """Split legacy <lora:name:strength> values into separate name/strength keys in-place."""
     for item in data.get(KEY_BATCH_DATA, []):
@@ -180,6 +189,7 @@ def load_json(path: str | Path) -> tuple[dict[str, Any], float]:
     try:
         with open(path, 'r') as f:
             data = json.load(f)
+        _ensure_default_keys(data)
         _migrate_lora_keys(data)
         return data, path.stat().st_mtime
     except Exception as e: