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/app.mjs

@@ -1191,7 +1191,10 @@ function render_grid_viewer() {
 	));
 
 	const cells_el = document.getElementById('grid-cells');
-	cells_el.style.gridTemplateColumns = `repeat(${grid.cols}, 1fr)`;
+	const gap = 4;
+	const available = cells_el.parentElement.clientWidth;
+	const cell_px = Math.floor((available - gap * (grid.cols - 1)) / grid.cols);
+	cells_el.style.gridTemplateColumns = `repeat(${grid.cols}, ${cell_px}px)`;
 
 	const all_cells = grid.cells.flat().map((filename, idx) => {
 		const row = Math.floor(idx / grid.cols);

public/style.css

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