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/i18n/packs/en.json

@@ -171,5 +171,13 @@
     "close_title": "Close",
     "section_close": "Close",
     "back_to_home": "← Back to Home"
+  },
+  "footer": {
+    "close": "Close",
+    "help": "Help",
+    "help_title": "DataTools",
+    "help_version": "Version {version}",
+    "help_support": "Support: {email}",
+    "help_dismiss": "Close"
   }
 }