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>

.streamlit/config.toml

@@ -0,0 +1,8 @@
+[client]
+toolbarMode = "minimal"
+
+[server]
+headless = true
+
+[browser]
+gatherUsageStats = false