Grid viewer: improve cell labels and add green inventory count badge

Labels now overlay image with gradient background and visible colour.
Green badge top-right shows number of inventory entries for that cell.

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

public/app.mjs

@@ -1294,6 +1294,18 @@ function render_grid_viewer() {
 			cell.classList.add('empty');
 		}
 		set_text(cell, '.grid-cell-label', `R${row + 1}C${col + 1}`);
+
+		const count = all_inventory.filter(inv =>
+			inv.location_type === 'grid' && inv.grid_id === grid.id &&
+			inv.grid_row === row && inv.grid_col === col
+		).length;
+		if (count > 0) {
+			const badge = document.createElement('span');
+			badge.className = 'grid-cell-count';
+			badge.textContent = count;
+			qs(cell, '.grid-cell-img-wrap').appendChild(badge);
+		}
+
 		cell.addEventListener('click', (e) => open_cell_inventory(grid, row, col, e));
 		return cell;
 	});