fix(nav,footer): drop orphan _hidden section header, show footer on Activate

Two follow-ups to the prior sidebar/footer cleanup:

- The "_hidden" section header was still visible in the sidebar
  because Streamlit renders ``stNavSectionHeader`` as a sibling of
  ``stNavSection``, not a child — so the ``:has()`` rule on the
  section was hiding the items list but leaving the header
  (and its collapse/drilldown marker) behind. Move Activate +
  Close into the unlabeled section (key ``""``) alongside Home so
  there is no header to leak in the first place, then hide just
  the two links via ``stSidebarNavLinkContainer:has(...)`` (with
  a defensive ``a[href$=...]`` fallback for browsers without
  ``:has()`` support).
- The sticky footer was missing on ``pages/_Activate.py`` because
  the page never called ``render_sticky_footer`` — added the
  call so the Help / Close bar persists when the user follows
  the popover's Activate / Manage link.

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

src/gui/app.py

@@ -109,13 +109,16 @@ def _build_navigation() -> dict[str, list]:
         url_path="close",
     )
 
+    # Activate + Close are placed in the unlabeled section alongside
+    # Home (key ``""`` — Streamlit renders no header for it). The CSS
+    # in ``hide_streamlit_chrome`` then hides just their two links by
+    # ``href``, leaving Home visible and no orphan section header /
+    # drilldown marker in the sidebar.
     return {
-        "": [home],
+        "": [home, activate, close],
         section_label("cleaners"): by_section["cleaners"],
         section_label("transformations"): by_section["transformations"],
         section_label("automations"): by_section["automations"],
-        # Hidden section — see comment above + CSS in hide_streamlit_chrome.
-        "_hidden": [activate, close],
     }