feat(footer): slim sticky footer with Close + Help, drop bottom Back-to-Home

The duplicate full-width Back-to-Home button at the bottom of every
tool page was reading as a "huge footer." Replace it with a real
slim sticky footer holding two controls:

- Close: <a href="./close"> to the Close page (which shuts down).
  Full-page nav is fine here — the process is terminating, so the
  session-state-loss concern that retired the previous sticky
  footer doesn't apply.
- Help: JS-toggled popover showing version + support@datatools.app.
  No navigation, no state loss.

Top-of-page Back-to-Home stays (uses st.switch_page, preserves
state). Add footer.* i18n keys for en + es.

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

src/gui/pages/3_Format_Standardizer.py

@@ -653,7 +653,6 @@ with dl_c:
         mime="application/json",
     )
 
-back_to_home_link(key="_back_to_home_link_bottom")
 
 st.divider()
 st.caption("Runs locally. Your data never leaves this computer. | DataTools v3.0")