fix(footer): restore soft-nav for Close (no page reload on shutdown)

Footer Close was using ``<a href="./close">`` which triggers a
browser hard-nav. That's a visible page-reload flash, websocket
churn, and slower shutdown than the previous sidebar Close —
which used ``st.navigation``'s soft nav.

Restore the soft-nav path:

- ``render_sticky_footer`` now renders a hidden ``st.page_link``
  pointing at ``pages/99_Close.py``. Positioned off-screen via
  CSS (``stElementContainer:has(a[data-testid=stPageLink]
  [href$=/close])``) so it occupies no layout space but stays in
  the DOM, reachable + clickable.
- Footer's Close <button> click handler now dispatches a
  programmatic click on that hidden page_link. Streamlit's React
  handler picks it up and runs the soft nav (same code path the
  old sidebar entry used). Falls back to ``window.location.href``
  if the helper link hasn't rendered yet so the button is never
  a no-op.
- The page_link call is wrapped in try/except: ``AppTest`` doesn't
  populate the page-nav session keys it needs and raises
  ``KeyError('url_pathname')``. Failure costs only the soft-nav
  optimization — Close still works via the hard-nav fallback.

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

src/gui/components/_legacy.py

@@ -728,11 +728,59 @@ def render_sticky_footer() -> None:
 #datatools-help-popover .dt-help-dismiss:hover {
     color: rgb(38, 39, 48) !important;
 }
+/* Hide the sticky-footer's helper st.page_link off-screen but
+   keep it in the DOM + clickable. The footer's Close button
+   dispatches a programmatic click on this link so navigation uses
+   Streamlit's soft nav (preserves the websocket, no visible page
+   reload) instead of the browser hard-nav an ``<a href="./close">``
+   would trigger. Off-screen (rather than ``display:none``) so
+   React event delegation works reliably across browsers. */
+[data-testid="stElementContainer"]:has(a[data-testid="stPageLink"][href$="/close"]),
+[data-testid="stElementContainer"]:has(a[data-testid="stPageLink"][href$="/close/"]) {
+    position: absolute !important;
+    left: -9999px !important;
+    top: -9999px !important;
+    width: 1px !important;
+    height: 1px !important;
+    overflow: hidden !important;
+    opacity: 0 !important;
+    pointer-events: none !important;
+}
+/* Defensive fallback for browsers without :has() — at least
+   shrink the inline page_link so it doesn't render a visible row. */
+a[data-testid="stPageLink"][href$="/close"],
+a[data-testid="stPageLink"][href$="/close/"] {
+    visibility: hidden !important;
+    height: 0 !important;
+    padding: 0 !important;
+    margin: 0 !important;
+}
 </style>
 """,
         unsafe_allow_html=True,
     )
 
+    # Hidden Streamlit page_link to the close page. The footer's
+    # Close button programmatically clicks the anchor this renders,
+    # which triggers Streamlit's soft navigation (same code path
+    # the previous sidebar Close entry used). The link is positioned
+    # off-screen via the CSS above so it doesn't take page space
+    # but remains reachable to the JS click dispatch.
+    #
+    # Wrapped because ``st.page_link`` raises ``KeyError('url_pathname')``
+    # under ``AppTest`` (the test harness does not populate the page-nav
+    # session keys ``page_link`` needs to mark itself active/inactive).
+    # The JS click handler has a hard-nav fallback when this helper
+    # link isn't present, so a failure here only costs the soft-nav
+    # optimization — Close still works.
+    try:
+        st.page_link(
+            "pages/99_Close.py",
+            label=_t("footer.close"),
+        )
+    except Exception:
+        pass
+
     from streamlit.components.v1 import html as _components_html
     _components_html(
         f"""
@@ -757,14 +805,32 @@ def render_sticky_footer() -> None:
       helpBtn.className = 'datatools-footer-btn help';
       helpBtn.textContent = labels.help;
 
-      var closeLink = doc.createElement('a');
-      closeLink.className = 'datatools-footer-btn close';
-      closeLink.href = './close';
-      closeLink.target = '_self';
-      closeLink.textContent = labels.close;
+      var closeBtn = doc.createElement('button');
+      closeBtn.type = 'button';
+      closeBtn.className = 'datatools-footer-btn close';
+      closeBtn.textContent = labels.close;
+      // Soft-nav via the hidden ``st.page_link`` that
+      // ``render_sticky_footer`` injects. Streamlit owns its click
+      // handler and will route through ``st.switch_page`` (same
+      // code path the old sidebar Close entry used) — no full-page
+      // reload, no websocket churn. Fall back to a hard nav if the
+      // helper link hasn't rendered yet (first paint race) so the
+      // button is never a no-op.
+      closeBtn.addEventListener('click', function (e) {{
+        e.preventDefault();
+        var helper = doc.querySelector(
+          'a[data-testid="stPageLink"][href$="/close"], ' +
+          'a[data-testid="stPageLink"][href$="/close/"]'
+        );
+        if (helper) {{
+          helper.click();
+        }} else {{
+          window.location.href = './close';
+        }}
+      }});
 
       div.appendChild(helpBtn);
-      div.appendChild(closeLink);
+      div.appendChild(closeBtn);
 
       var pop = doc.createElement('div');
       pop.id = 'datatools-help-popover';