fix(gui): keep sidebar reopenable + add clean Quit button

The chrome-hiding CSS was removing the Streamlit header wholesale,
which also took the sidebar's expand chevron with it — a collapsed
sidebar became unreopenable. Make the header transparent instead and
explicitly preserve the sidebar collapsed-control.

Also add a Quit button in the app footer that signals the Streamlit
server (SIGTERM, falling back to SIGINT) so closing the GUI returns
the shell prompt cleanly instead of leaving Python hung.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/app.py

@@ -24,6 +24,7 @@ if str(_project_root) not in sys.path:
 from src.gui.components import (
     findings_count_for_tool,
     hide_streamlit_chrome,
+    quit_button,
     upload_and_analyze_section,
 )
 
@@ -87,7 +88,11 @@ for row_start in range(0, len(TOOLS), 3):
 # ---------------------------------------------------------------------------
 
 st.divider()
-st.caption(
-    "Runs locally. Your data never leaves this computer. "
-    "| DataTools v3.0"
-)
+footer_left, footer_right = st.columns([4, 1])
+with footer_left:
+    st.caption(
+        "Runs locally. Your data never leaves this computer. "
+        "| DataTools v3.0"
+    )
+with footer_right:
+    quit_button()