test(footer): move marker out of footer into main content flow

User confirmed the previous marker landed inside the Help/Close
sticky footer — which is NOT the offending white bar. They want the
sticky footer kept; the white strip they're trying to remove sits
*above* the footer in the main content area.

Move the marker out of ``#datatools-sticky-footer`` and render it
via ``st.markdown`` immediately before the ``st.iframe`` call that
injects the footer. That places it at the very bottom of
``stAppViewBlockContainer`` — exactly where the iframe wrapper
(``stElementContainer``) and the block container's
``padding-bottom: 3rem`` reservation live.

Styled as a red dashed banner so it's unmistakable. If it lines up
with the white strip clipping text on scroll, one of those two is
the culprit and the next commit can target it.

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

src/gui/components/_legacy.py

@@ -787,6 +787,22 @@ a[data-testid="stPageLink-NavLink"][href*="close"] {
     except Exception:
         pass
 
+    # TEMP: visible marker in the main content flow, immediately before
+    # the footer-injection iframe. This lands at the very bottom of
+    # ``stAppViewBlockContainer`` — right where the user reports a
+    # white strip clipping text when scrolling. If this red bar lines
+    # up with the offending strip, the strip is the iframe wrapper
+    # (``stElementContainer`` around ``st.iframe``) or the block
+    # container's ``padding-bottom: 3rem`` reservation.
+    st.markdown(
+        '<div style="background:#ffe0e0;border:2px dashed #b00020;'
+        'color:#b00020;font-weight:700;font-size:13px;text-align:center;'
+        'padding:8px 12px;margin:4px 0;font-family:system-ui,sans-serif;">'
+        '◀ CLAUDE TEST #2 — IS THIS THE WHITE BAR? ▶'
+        '</div>',
+        unsafe_allow_html=True,
+    )
+
     st.iframe(
         f"""
 <script>
@@ -814,16 +830,6 @@ a[data-testid="stPageLink-NavLink"][href*="close"] {
       closeBtn.type = 'button';
       closeBtn.className = 'datatools-footer-btn close';
       closeBtn.textContent = labels.close;
-
-      // TEMP: visible marker so the user can confirm whether
-      // ``#datatools-sticky-footer`` is the "white bar" they want
-      // removed. To be deleted once identification is confirmed.
-      var marker = doc.createElement('span');
-      marker.textContent =
-        '◀ CLAUDE TEST: is this the white bar you want removed? ▶';
-      marker.style.cssText =
-        'color:#b00020;font-weight:700;font-size:12px;'
-        + 'margin-left:0.6rem;font-family:system-ui,sans-serif;';
       // 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
@@ -858,7 +864,6 @@ a[data-testid="stPageLink-NavLink"][href*="close"] {
 
       div.appendChild(helpBtn);
       div.appendChild(closeBtn);
-      div.appendChild(marker);
 
       var pop = doc.createElement('div');
       pop.id = 'datatools-help-popover';