Fix grid viewer zoom: fixed cell px size + overflow-x scroll

Cells are sized in CSS px to fit at load time. Browser zoom now scales
everything uniformly; grid scrolls horizontally instead of reflowing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/style.css

@@ -1338,6 +1338,10 @@ nav {
 }
 
 
+.grid-viewer {
+	overflow-x: auto;
+}
+
 .grid-cells {
 	display: grid;
 	gap: 4px;