Add zoom/cell-size slider to grid viewer

Images and labels scale together with a range slider (40–300px).
Grid scrolls horizontally when cells exceed viewport width.

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

public/app.mjs

@@ -1190,9 +1190,16 @@ function render_grid_viewer() {
 		}
 	));
 
-	// Build cell grid
+	// Zoom slider
+	const zoom_el = document.getElementById('gv-zoom');
 	const cells_el = document.getElementById('grid-cells');
-	cells_el.style.gridTemplateColumns = `repeat(${grid.cols}, 1fr)`;
+
+	function apply_zoom(px) {
+		cells_el.style.gridTemplateColumns = `repeat(${grid.cols}, ${px}px)`;
+	}
+
+	zoom_el.addEventListener('input', () => apply_zoom(zoom_el.value));
+	apply_zoom(zoom_el.value);
 
 	const all_cells = grid.cells.flat().map((filename, idx) => {
 		const row = Math.floor(idx / grid.cols);