chore: 初始化 BlockFlow Workbench 仓库

建立前端与 Tauri 桌面端的首个版本提交，包含核心编辑器、项目文件读写、测试与构建配置。

补充 Git 忽略规则和换行规范，排除依赖、构建产物、本地运行日志与临时验证文件，方便在其他电脑继续开发。

src/editor/extensions/SelectedBlockExtension.ts

@@ -0,0 +1,61 @@
+import { Extension } from "@tiptap/core"
+import { Plugin, PluginKey } from "@tiptap/pm/state"
+import { Decoration, DecorationSet } from "@tiptap/pm/view"
+import type { Node as ProseMirrorNode } from "@tiptap/pm/model"
+
+export const selectedBlockPluginKey = new PluginKey<DecorationSet>("blockFlowSelectedBlock")
+
+export const SelectedBlockExtension = Extension.create({
+  name: "blockFlowSelectedBlock",
+
+  addStorage() {
+    return {
+      selectedBlockId: null as string | null
+    }
+  },
+
+  addProseMirrorPlugins() {
+    return [
+      new Plugin({
+        key: selectedBlockPluginKey,
+        state: {
+          init: (_, state) => buildSelectedBlockDecorations(state.doc, this.storage.selectedBlockId as string | null),
+          apply: (transaction, previous, _oldState, newState) => {
+            const selectedBlockId = transaction.getMeta(selectedBlockPluginKey) as string | null | undefined
+            if (selectedBlockId === undefined && !transaction.docChanged) {
+              return previous
+            }
+
+            return buildSelectedBlockDecorations(
+              newState.doc,
+              selectedBlockId === undefined ? (this.storage.selectedBlockId as string | null) : selectedBlockId
+            )
+          }
+        },
+        props: {
+          decorations(state) {
+            return selectedBlockPluginKey.getState(state)
+          }
+        }
+      })
+    ]
+  }
+})
+
+function buildSelectedBlockDecorations(doc: ProseMirrorNode, selectedBlockId: string | null): DecorationSet {
+  if (selectedBlockId === null) {
+    return DecorationSet.empty
+  }
+
+  const decorations: Decoration[] = []
+  doc.descendants((node, position) => {
+    if (node.attrs.id === selectedBlockId) {
+      decorations.push(Decoration.node(position, position + node.nodeSize, { class: "bf-selected-block" }))
+      return false
+    }
+
+    return true
+  })
+
+  return DecorationSet.create(doc, decorations)
+}