fix(footer,nav): left-justify buttons, drop per-page caption bar, hide sidebar Close

Three small follow-ups to the sticky-footer rework:

- Left-justify the footer buttons (and reposition the Help popover
  to anchor at the left edge so it lines up with its trigger).
- Remove the per-page ``st.divider() + st.caption("Runs locally…")``
  trailing block from all 9 tool pages. The new sticky footer
  covers that text, so it was rendering as an empty white bar at
  the bottom of each tool page.
- Hide the Close entry from the sidebar nav via CSS. The page stays
  registered with st.navigation so /close is still routable for the
  sticky-footer Close button — only the sidebar link + its section
  header are hidden (via :has() on stNavSection).

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

src/gui/pages/6_Outlier_Detector.py

@@ -97,14 +97,3 @@ st.selectbox("Action", ["Flag only (add column)", "Remove outlier rows", "Cap /
 st.divider()
 st.button("Detect Outliers", type="primary", use_container_width=True, disabled=True)
 
-# ---------------------------------------------------------------------------
-# Footer
-# ---------------------------------------------------------------------------
-
-
-st.divider()
-st.caption(
-    "Runs locally. Your data never leaves this computer. "
-    "| DataTools v3.0"
-)
-