feat(canvas): 画布背景 - 默认网格由点阵改为线条

新建画布、读取未指定背景的画布、公开分享页，以及 InfiniteCanvas 组件自身的 prop
默认值，统一改为 lines。

四处默认值必须一起改：store 的新建与恢复分支各一份、分享页一份、组件 prop 默认一份。
漏掉任意一处就会出现「自己的画布是线条、分享出去变点阵」这类不报错的不一致。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/src/components/canvas/infinite-canvas.tsx

@@ -39,7 +39,7 @@ type PinchState = {
     initialScale: number;
 };
 
-export function InfiniteCanvas({ containerRef, viewport, backgroundMode = "dots", onViewportChange, onViewportPreviewChange, onCanvasMouseDown, boxSelectEnabled = false, onCanvasDoubleClick, onCanvasDeselect, onContextMenu, onDrop, onFileDragEnter, onFileDragLeave, onFileDragOver, graphicsLayer, children }: InfiniteCanvasProps) {
+export function InfiniteCanvas({ containerRef, viewport, backgroundMode = "lines", onViewportChange, onViewportPreviewChange, onCanvasMouseDown, boxSelectEnabled = false, onCanvasDoubleClick, onCanvasDeselect, onContextMenu, onDrop, onFileDragEnter, onFileDragLeave, onFileDragOver, graphicsLayer, children }: InfiniteCanvasProps) {
     const theme = canvasThemes[useThemeStore((state) => state.theme)];
     const panState = useRef({
         isPanning: false,