feat(exams,homework): add rich text exam editor and scan-based grading

- Add Tiptap-based rich text editor with custom extensions (dotted-mark,
  blank-node, image-node, group-block, question-block) for exam creation
- Add AI auto-marking action to convert pasted exam text to structured editor doc
- Add resizable split-panel layout for editor + live preview
- Add student scan upload (photo of paper answers) with drag-drop and reorder
- Add scan image viewer with zoom/rotate/fullscreen for teachers
- Add scan grading view with side-by-side questions and scan images
- Add /teacher/exams/new and /teacher/homework/submissions/[id]/scan-grading routes
- Fix getScansAction to support both teacher (HOMEWORK_GRADE) and student
  (HOMEWORK_SUBMIT) permission scopes
- Add i18n keys for rich editor, scan upload, and scan grading (zh-CN/en)
- Sync architecture diagrams (004/005) with new modules, routes, and deps

src/modules/exams/editor/selection-toolbar.tsx

@@ -0,0 +1,202 @@
+"use client"
+
+import { useEffect, useState } from "react"
+import type { Editor } from "@tiptap/react"
+import {
+  Box,
+  Brackets,
+  CircleSlash,
+  FileText,
+  Heading,
+  Underline,
+} from "lucide-react"
+
+import { cn } from "@/shared/lib/utils"
+import { Button } from "@/shared/components/ui/button"
+import type { QuestionBlockType } from "./extensions/question-block"
+
+interface SelectionToolbarProps {
+  editor: Editor | null
+  /** 当前选区所在题目块类型(用于显示状态) */
+  activeQuestionType?: QuestionBlockType
+  className?: string
+}
+
+interface ToolbarButtonProps {
+  onClick: () => void
+  icon: React.ElementType
+  label: string
+  active?: boolean
+}
+
+function ToolbarButton({ onClick, icon: Icon, label, active }: ToolbarButtonProps) {
+  return (
+    <Button
+      type="button"
+      variant={active ? "secondary" : "ghost"}
+      size="sm"
+      className={cn(
+        "h-7 gap-1 px-2 text-xs",
+        active && "bg-primary/15 text-primary hover:bg-primary/20"
+      )}
+      onMouseDown={(e) => {
+        // 防止点击按钮时编辑器失焦
+        e.preventDefault()
+      }}
+      onClick={(e) => {
+        e.preventDefault()
+        onClick()
+      }}
+      title={label}
+    >
+      <Icon className="h-3.5 w-3.5" />
+      <span>{label}</span>
+    </Button>
+  )
+}
+
+/**
+ * 选区标记浮层 —— 选中文本时浮现,提供"标记为题目/分组/加点字/填空"等快捷操作。
+ * 跟随选区位置定位,使用 getBoundingClientRect 计算坐标。
+ */
+export function SelectionToolbar({
+  editor,
+  activeQuestionType,
+  className,
+}: SelectionToolbarProps) {
+  const [coords, setCoords] = useState<{ top: number; left: number } | null>(null)
+  const [hasSelection, setHasSelection] = useState(false)
+
+  useEffect(() => {
+    if (!editor) return
+
+    const updatePosition = () => {
+      const { from, to, empty } = editor.state.selection
+      if (empty || from === to) {
+        setCoords(null)
+        setHasSelection(false)
+        return
+      }
+
+      // 检查选区是否在 questionBlock/groupBlock 内部
+      const $from = editor.state.selection.$from
+      const depth = $from.depth
+      let inBlock = false
+      for (let d = depth; d > 0; d--) {
+        const node = $from.node(d)
+        if (node.type.name === "questionBlock" || node.type.name === "groupBlock") {
+          inBlock = true
+          break
+        }
+      }
+
+      // 即使不在块内也允许显示(可标记为题目/分组)
+      void inBlock
+
+      const view = editor.view
+      const start = view.coordsAtPos(from)
+      const end = view.coordsAtPos(to)
+      const top = Math.min(start.top, end.top) - 44 // 浮在选区上方
+      const left = (start.left + end.right) / 2
+      setCoords({ top, left })
+      setHasSelection(true)
+    }
+
+    editor.on("selectionUpdate", updatePosition)
+    editor.on("blur", () => {
+      // 延迟以允许点击工具栏按钮
+      setTimeout(() => {
+        if (!editor.isFocused) {
+          setCoords(null)
+          setHasSelection(false)
+        }
+      }, 200)
+    })
+
+    return () => {
+      editor.off("selectionUpdate", updatePosition)
+    }
+  }, [editor])
+
+  if (!editor || !hasSelection || !coords) return null
+
+  const insertQuestion = (type: QuestionBlockType) => {
+    editor.chain().focus().insertQuestion({ type, score: type === "composite" ? 5 : 2 }).run()
+  }
+
+  const insertGroup = () => {
+    editor.chain().focus().insertGroup("一、选择题").run()
+  }
+
+  const toggleDotted = () => {
+    editor.chain().focus().toggleDotted().run()
+  }
+
+  const insertBlank = () => {
+    editor.chain().focus().insertBlank().run()
+  }
+
+  const insertImage = () => {
+    // 触发隐藏的文件输入
+    const input = document.createElement("input")
+    input.type = "file"
+    input.accept = "image/*"
+    input.onchange = async () => {
+      const file = input.files?.[0]
+      if (!file) return
+      const formData = new FormData()
+      formData.append("file", file)
+      formData.append("targetType", "exam")
+      try {
+        const res = await fetch("/api/upload", { method: "POST", body: formData })
+        const data = await res.json()
+        if (data?.success && data?.url) {
+          editor.chain().focus().setImage({ src: data.url, fileId: data.id }).run()
+        }
+      } catch (e) {
+        console.error("[SelectionToolbar] upload failed", e)
+      }
+    }
+    input.click()
+  }
+
+  return (
+    <div
+      role="toolbar"
+      aria-label="选区标记工具栏"
+      className={cn(
+        "fixed z-50 flex items-center gap-0.5 rounded-md border bg-popover/95 p-1 shadow-md backdrop-blur",
+        className
+      )}
+      style={{
+        top: `${Math.max(8, coords.top)}px`,
+        left: `${coords.left}px`,
+        transform: "translateX(-50%)",
+      }}
+    >
+      <ToolbarButton onClick={insertGroup} icon={Heading} label="大题" />
+      <ToolbarButton
+        onClick={() => insertQuestion("single_choice")}
+        icon={CircleSlash}
+        label="单选"
+        active={activeQuestionType === "single_choice"}
+      />
+      <ToolbarButton
+        onClick={() => insertQuestion("text")}
+        icon={FileText}
+        label="填空/简答"
+        active={activeQuestionType === "text"}
+      />
+      <ToolbarButton
+        onClick={() => insertQuestion("composite")}
+        icon={Brackets}
+        label="复合"
+        active={activeQuestionType === "composite"}
+      />
+      <div className="mx-1 h-5 w-px bg-border" />
+      <ToolbarButton onClick={toggleDotted} icon={Underline} label="加点字" />
+      <ToolbarButton onClick={insertBlank} icon={Box} label="填空" />
+      <ToolbarButton onClick={insertImage} icon={FileText} label="图片" />
+    </div>
+  )
+}