Footer bars: extract standalone slices, shrink Stanford Medicine logo

The legacy nav_banners.png sprite was leaking content (the 'Group Member Site'
header and institutional-logo strip) past the right edge of each bar at
container widths > the slice's natural width. Cropped two clean PNGs from
the sprite — footer_bar_dark.png (599x39) and footer_bar_teal.png (205x39) —
and use background-size: 100% 100% to stretch them to the flex container's
width. Same fix applied to .home-divider-bars on the home page.

Stanford Medicine combined logo shrunk from height 80 -> 40 (50% smaller)
to balance against the smaller HHMI mark.

Author: rhiju

_includes/footer.html

@@ -5,7 +5,7 @@
   </div>
   <div class="institutional-logos">
     <a href="https://med.stanford.edu/" target="_blank" rel="noopener noreferrer external">
-      <img src="{{ '/assets/images/site/public/nav_logo_stanford_medicine.png' | relative_url }}" alt="Stanford Medicine" height="80">
+      <img src="{{ '/assets/images/site/public/nav_logo_stanford_medicine.png' | relative_url }}" alt="Stanford Medicine" height="40">
     </a>
     <a href="https://www.hhmi.org/scientists/rhiju-das" target="_blank" rel="noopener noreferrer external">
       <img src="{{ '/assets/images/site/public/nav_logo_hhmi.jpg' | relative_url }}" alt="Howard Hughes Medical Institute" height="60">

assets/css/style.scss

@@ -282,10 +282,17 @@ hr { border: 0; border-top: 1px dotted $text-faint; margin: 2rem 0; }
 .home-divider-bars .bar-dark,
 .home-divider-bars .bar-light {
   height: 39px;
-  background: url("{{ '/assets/images/site/public/nav_banners.png' | relative_url }}") no-repeat;
+  background-repeat: no-repeat;
+  background-size: 100% 100%;
+}
+.home-divider-bars .bar-dark  {
+  flex: 0 0 60%;
+  background-image: url("{{ '/assets/images/site/public/footer_bar_dark.png' | relative_url }}");
+}
+.home-divider-bars .bar-light {
+  flex: 1 1 auto;
+  background-color: #ccc;
 }
-.home-divider-bars .bar-dark  { flex: 0 0 60%; background-position: 0 -15px; }
-.home-divider-bars .bar-light { flex: 1 1 auto; background-position: -514px -55px; }
 
 .home-callouts {
   display: grid;
@@ -720,14 +727,20 @@ details.show-more {
 .footer-bar-dark,
 .footer-bar-teal {
   height: 39px;
-  background-image: url("{{ '/assets/images/site/public/nav_banners.png' | relative_url }}");
   background-repeat: no-repeat;
+  background-size: 100% 100%;
+}
+/* Standalone slices extracted from nav_banners.png so each bar is just its
+   own clean image (no leakage of the 'Group Member Site' header or the
+   institutional-logo strip from the source sprite). */
+.footer-bar-dark {
+  flex: 0 0 70%;
+  background-image: url("{{ '/assets/images/site/public/footer_bar_dark.png' | relative_url }}");
+}
+.footer-bar-teal {
+  flex: 1 1 auto;
+  background-image: url("{{ '/assets/images/site/public/footer_bar_teal.png' | relative_url }}");
 }
-/* Sprite slices from nav_banners.png. The dark gradient (footer_left) lives at
-   y=15-54, teal block (footer_right) at x=0-256, y=55-94. Ratio ~70/30 to
-   align with the hero banner's photo/tab split. */
-.footer-bar-dark { flex: 0 0 70%; background-position: 0 -15px; background-size: auto 39px; }
-.footer-bar-teal { flex: 1 1 auto; background-position: 0 -55px; background-size: auto 39px; }
 
 /* Home page already has decorative bars between hero and content; suppress the
    footer pair so the home doesn't look top-heavy with bars. */