fix: early return in _load_deck_cards_by_name when deck not found in DB

Fixes UnboundLocalError crash when deck_row is None by returning an
empty list instead of falling through to a query with an unbound deck_id.

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

services/deck_service.py

@@ -178,7 +178,8 @@ def _load_deck_cards_by_name(deck_name: str) -> list:
     """Load cards from a deck by name, returning list of card dicts."""
     conn = _get_db_conn()
     deck_row = conn.execute("SELECT id FROM decks WHERE name = ?", (deck_name,)).fetchone()
-    if deck_row:
+    if not deck_row:
+        return []
     deck_id = deck_row[0]
     rows = conn.execute("""
         SELECT dc.card_name, dc.quantity, dc.is_commander, dc.role_tag, dc.scryfall_id,