feat(DATA-004): add underlying instrument selector

tests/test_settings_page.py

@@ -4,6 +4,23 @@ from app.models.portfolio import PortfolioConfig
 from app.pages.settings import _save_card_status_text
 
 
+def test_portfolio_config_underlying_defaults_to_gld() -> None:
+    """Verify PortfolioConfig underlying field defaults to GLD."""
+    config = PortfolioConfig(gold_value=215_000.0, entry_price=215.0, loan_amount=145_000.0)
+    assert config.underlying == "GLD"
+
+
+def test_portfolio_config_underlying_can_be_set_to_gc_f() -> None:
+    """Verify PortfolioConfig underlying can be set to GC=F."""
+    config = PortfolioConfig(
+        gold_value=215_000.0,
+        entry_price=215.0,
+        loan_amount=145_000.0,
+        underlying="GC=F",
+    )
+    assert config.underlying == "GC=F"
+
+
 def test_save_card_status_text_for_clean_state() -> None:
     config = PortfolioConfig(gold_value=215_000.0, entry_price=215.0, loan_amount=145_000.0)