Add Avery 5260 label template

Same 30-per-sheet, 1" x 2-5/8" grid as 5160 (rounded corners are the
only difference), so unlike 61526 this one isn't flagged approximate.

internal/web/handlers_labels.go

@@ -37,6 +37,14 @@ var labelTemplates = map[string]LabelTemplate{
 		MarginTopIn: 0.5, MarginLeftIn: 0.1875, ColGapIn: 0.125, RowGapIn: 0,
 		QRSizeIn: 0.85,
 	},
+	"5260": {
+		// Same 30-per-sheet grid as 5160 (1" x 2-5/8"); 5260 is just the
+		// rounded-corner version of the same physical layout.
+		Key: "5260", DisplayName: `Avery 5260 - 1" x 2-5/8" (30/sheet, rounded corners)`,
+		LabelWidthIn: 2.625, LabelHeightIn: 1, Cols: 3, Rows: 10,
+		MarginTopIn: 0.5, MarginLeftIn: 0.1875, ColGapIn: 0.125, RowGapIn: 0,
+		QRSizeIn: 0.85,
+	},
 	"61526": {
 		Key: "61526", DisplayName: `Asset Tag Labels 61526 - 3/4" x 2" (~48/sheet)`,
 		LabelWidthIn: 2, LabelHeightIn: 0.75, Cols: 4, Rows: 12,
@@ -152,7 +160,7 @@ func (a *App) handleLabelsPage(w http.ResponseWriter, r *http.Request) {
 
 	a.Render(w, http.StatusOK, "labels", labelsPageData{
 		User: currentUser(r), Labels: labels, SheetCSS: tmpl.css(),
-		Templates: labelTemplates, TemplateKeys: []string{"5160", "61526"},
+		Templates: labelTemplates, TemplateKeys: []string{"5160", "5260", "61526"},
 		Selected: tmpl, AssetID: assetIDParam, FolderID: folderIDParam, Recursive: recursive,
 	})
 }