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/7_Multi_File_Merger.py

@@ -99,7 +99,6 @@ st.button("Merge Files", type="primary", use_container_width=True, disabled=True
 # Footer
 # ---------------------------------------------------------------------------
 
-back_to_home_link(key="_back_to_home_link_bottom")
 
 st.divider()
 st.caption(