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/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 || [],