fix(footer): tighten block-container `padding-bottom` to close white gap

Diagnostics confirmed the "white bar" the user has been describing is
not a separate element — it's ``[data-testid=stApp]``'s solid white
background (``rgb(255,255,255)``, viewport-locked) showing through the
gap between where page content ends and where the fixed Help/Close
footer overlay begins. ``stApp`` stays put while content scrolls
inside it, which is why the bar "doesn't change when scrolling".

The gap exists because ``render_sticky_footer`` overrides the block
container's ``padding-bottom`` to ``3rem`` (48px) to reserve clear
room for the fixed footer. The footer is only ~32-33px tall (min-
height 32px + 0.25rem top/bottom padding), so ~16px of that reserve
was pure visible white space sitting above the buttons.

Reduce ``padding-bottom`` to ``2rem`` (~32px) — just enough to
prevent content from rendering under the footer overlay, no more.
Eliminates the visible gap without exposing text to clipping.

Also remove the diagnostic banner + click-to-inspect iframe from
the home page now that the bar is identified.

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

src/gui/components/_legacy.py

@@ -629,7 +629,12 @@ def render_sticky_footer() -> None:
         """
 <style>
 [data-testid="stAppViewBlockContainer"] {
-    padding-bottom: 3rem !important;
+    /* Reserve just enough room for the fixed footer overlay (min-height
+       32px + 0.25rem * 2 padding ≈ 2rem). Anything larger leaves a
+       visible white gap above the footer that the user sees as a
+       "white bar" between content and the Help/Close row — actually
+       just ``.stApp``'s solid-white background showing through. */
+    padding-bottom: 2rem !important;
 }
 #datatools-sticky-footer {
     position: fixed !important;