leaderboard: widen gallery chrome reserve to kill residual outer scroll

Live verification showed a constant ~56px outer-page overflow on smaller
screens (real HF chrome is a bit taller than the harness estimate). Bump
CHROME_RESERVE 560->620 and lower the box floor so the whole gallery fits
the viewport across screen sizes (single scroll).

Co-authored-by: Cursor <cursoragent@cursor.com>

gallery.py

@@ -600,11 +600,11 @@ function syncHeadHeight() {
 // rather than the box plus an outer page scroll. Deliberately generous: a box
 // that is a little short (a touch more in-box scrolling) is far better than a
 // confusing second scrollbar.
-var CHROME_RESERVE = 560;
+var CHROME_RESERVE = 620;
 function sizeGalleryBox() {
   try {
     const avail = (window.screen && window.screen.availHeight) || 900;
-    const h = Math.max(300, Math.min(760, Math.round(avail - CHROME_RESERVE)));
+    const h = Math.max(280, Math.min(760, Math.round(avail - CHROME_RESERVE)));
     document.documentElement.style.setProperty('--gallery-max', h + 'px');
   } catch (e) { /* keep CSS fallback */ }
 }