fix(layout): bump bottom block-container padding 4rem → 7rem

Last lines on long tool pages were still grazing the fixed Help/Close
footer when scrolled all the way down. 4rem gave the cursor of free
space the footer claims but no breathing room — the bottom button
or text was visually flush against the footer's top edge. 7rem buys
~3rem of clear space on every page so the last content row reads
without obstruction.

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

src/gui/components/_legacy.py

@@ -104,11 +104,13 @@ footer {
 .stMainBlockContainer,
 [data-testid="stMainBlockContainer"] {
     padding-top: 0.5rem !important;
-    /* +4rem buys a comfortable read of the last line of content on a
-       page that fills the viewport: the fixed Help/Close footer is
-       ~36px (≈2.25rem) tall and we want at least a button-height of
-       free space between the last widget and the footer's top edge. */
-    padding-bottom: 4rem !important;
+    /* The fixed Help/Close footer sits over the bottom of the
+       viewport (~36px tall) — the block container needs enough
+       bottom padding that the LAST line of content clears it cleanly
+       when scrolled all the way down. 4rem turned out to be just
+       enough to graze the footer's top; 7rem buys clear breathing
+       room on every page including the long tool pages. */
+    padding-bottom: 7rem !important;
 }
 /* Scale content to fit app window */
 .stApp {