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/app.py

@@ -21,17 +21,15 @@ if str(_project_root) not in sys.path:
 # Page config
 # ---------------------------------------------------------------------------
 
+from src.gui.components import hide_streamlit_chrome
+
 st.set_page_config(
     page_title="DataTools — Data Cleaning Mastery",
     page_icon="🧹",
     layout="wide",
 )
 
-# Hide the Deploy button from the toolbar
-st.markdown(
-    "<style>[data-testid='stAppDeployButton'] {display: none;}</style>",
-    unsafe_allow_html=True,
-)
+hide_streamlit_chrome()
 
 
 # ---------------------------------------------------------------------------