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,

web/src/pages/canvas/shared.tsx

@@ -34,7 +34,7 @@ export default function SharedCanvasPage() {
     const [title, setTitle] = useState("共享画布");
     const [nodes, setNodes] = useState<CanvasNodeData[]>([]);
     const [connections, setConnections] = useState<Awaited<ReturnType<typeof getPublicCanvasShare>>["project"]["connections"]>([]);
-    const [backgroundMode, setBackgroundMode] = useState<"lines" | "dots" | "blank">("dots");
+    const [backgroundMode, setBackgroundMode] = useState<"lines" | "dots" | "blank">("lines");
     const [viewport, setViewport] = useState<ViewportTransform>({ x: 0, y: 0, k: 1 });
     const [selectedNodeId, setSelectedNodeId] = useState<string | null>(null);
     const [infoNodeId, setInfoNodeId] = useState<string | null>(null);
@@ -80,7 +80,7 @@ export default function SharedCanvasPage() {
             setTitle(project.title || "共享画布");
             setNodes((project.nodes || []).map(ensureMediaNodeMinimumSize));
             setConnections(project.connections || []);
-            setBackgroundMode(project.backgroundMode || "dots");
+            setBackgroundMode(project.backgroundMode || "lines");
             const initial = project.viewport || { x: 0, y: 0, k: 1 };
             viewportRef.current = initial;
             setViewport(initial);

web/src/stores/canvas/use-canvas-store.ts

@@ -328,7 +328,7 @@ export const useCanvasStore = create<CanvasStore>()(
                     connections: [],
                     chatSessions: [],
                     activeChatId: null,
-                    backgroundMode: "dots",
+                    backgroundMode: "lines",
                     showImageInfo: false,
                     viewport: initialViewport,
                     directorScenes: [],
@@ -348,7 +348,7 @@ export const useCanvasStore = create<CanvasStore>()(
                     connections: source.connections || [],
                     chatSessions: source.chatSessions || [],
                     activeChatId: source.activeChatId || null,
-                    backgroundMode: source.backgroundMode || "dots",
+                    backgroundMode: source.backgroundMode || "lines",
                     showImageInfo: source.showImageInfo || false,
                     viewport: source.viewport || initialViewport,
                     directorScenes: source.directorScenes || [],