feat(i18n): add language-pack scaffold with English and Spanish

Introduces ``src/i18n`` with a tiny JSON-backed t() lookup, an in-session
language preference, and a sidebar selector wired through
``hide_streamlit_chrome`` so every page picks up the same picker. Covers
home, tool cards, findings panel, gate, shutdown, and pickup banner
strings. Tests pin pack parity and the farewell-overlay JS escape so
future packs can't silently regress.

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

src/gui/pages/99_Close.py

@@ -18,24 +18,21 @@ if str(_project_root) not in sys.path:
     sys.path.insert(0, str(_project_root))
 
 from src.gui.components import hide_streamlit_chrome, quit_button
+from src.i18n import t
 
 st.set_page_config(
-    page_title="DataTools — Close",
+    page_title=t("close_page.page_title"),
     page_icon="🛑",
     layout="wide",
 )
 
 hide_streamlit_chrome()
 
-st.title("🛑 Close DataTools")
-st.caption("Shut down the local app and free the terminal.")
+st.title(t("close_page.title"))
+st.caption(t("close_page.caption"))
 st.divider()
 
-st.markdown(
-    "Clicking the button below will terminate the DataTools server. "
-    "Any unsaved work in other tools will be lost. Once the app shuts "
-    "down you can close this window."
-)
+st.markdown(t("close_page.body"))
 
 st.write("")
-quit_button(label="Close the app", key="quit_app_button_page")
+quit_button(label=t("close_page.button"), key="quit_app_button_page")