test(footer): inject visible marker into #datatools-sticky-footer

The user reports a "white bar/box" at the bottom of the main content
area that clips text when scrolling. The DOM inspector found only one
fixed-position white element near the viewport bottom —
``#datatools-sticky-footer`` (bg ``rgba(255,255,255,0.97)``,
~33px tall) — so this is my best candidate for what they're seeing.

Append a red marker span "◀ CLAUDE TEST: is this the white bar you
want removed? ▶" inside the footer div so the user can visually
confirm. If the text shows up where they see the offending white
bar, the footer is the right target; if the bar is somewhere else,
this confirms it's a different element.

Temporary — to be reverted in the next commit either way.

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

src/gui/components/_legacy.py

@@ -814,6 +814,16 @@ 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
@@ -848,6 +858,7 @@ 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';