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/components.py

@@ -1,4 +1,4 @@
-"""Reusable Streamlit widgets for the deduplicator GUI."""
+"""Reusable Streamlit widgets for the DataTools GUI."""
 
 from __future__ import annotations
 
@@ -24,6 +24,45 @@ from src.core.config import (
 from src.core.normalizers import NormalizerType
 
 
+# ---------------------------------------------------------------------------
+# App chrome — hide Streamlit default UI for app-like feel
+# ---------------------------------------------------------------------------
+
+_HIDE_CHROME_CSS = """
+<style>
+/* Hide Streamlit header bar */
+header[data-testid="stHeader"] {
+    display: none !important;
+}
+/* Hide hamburger menu */
+button[kind="header"] {
+    display: none !important;
+}
+#MainMenu {
+    display: none !important;
+}
+/* Hide footer */
+footer {
+    display: none !important;
+}
+/* Hide deploy button */
+[data-testid="stAppDeployButton"] {
+    display: none !important;
+}
+/* Reclaim top padding lost from hidden header */
+.stAppViewBlockContainer,
+[data-testid="stAppViewBlockContainer"] {
+    padding-top: 1rem !important;
+}
+</style>
+"""
+
+
+def hide_streamlit_chrome() -> None:
+    """Inject CSS to hide Streamlit's default header, menu, and footer."""
+    st.markdown(_HIDE_CHROME_CSS, unsafe_allow_html=True)
+
+
 # ---------------------------------------------------------------------------
 # Config panel (advanced options)
 # ---------------------------------------------------------------------------