feat: hide Streamlit chrome for app-like appearance

Add shared hide_streamlit_chrome() helper that removes header bar,
hamburger menu, footer, and deploy button via CSS injection. Called
on every page. Add .streamlit/config.toml with minimal toolbar mode.

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

src/gui/pages/4_Missing_Values.py

@@ -11,6 +11,10 @@ _project_root = Path(__file__).resolve().parent.parent.parent.parent
 if str(_project_root) not in sys.path:
     sys.path.insert(0, str(_project_root))
 
+from src.gui.components import hide_streamlit_chrome
+
+hide_streamlit_chrome()
+
 # ---------------------------------------------------------------------------
 # Header
 # ---------------------------------------------------------------------------