Add assignment regrade update flow

frontend/app/page.tsx

@@ -40,6 +40,7 @@ export default function HomePage() {
   const [assignments, setAssignments] = useState<AssignmentSummary[]>([]);
   const [selectedId, setSelectedId] = useState<number | null>(null);
   const [selected, setSelected] = useState<AssignmentDetail | null>(null);
+  const [editingId, setEditingId] = useState<number | null>(null);
 
   const [title, setTitle] = useState("");
   const [images, setImages] = useState<ImageItem[]>([]);
@@ -255,6 +256,23 @@ export default function HomePage() {
     );
   };
 
+  const beginEdit = async (item: AssignmentSummary) => {
+    setError("");
+    clearImages();
+    setTitle(item.title);
+    setEditingId(item.id);
+    setSelectedId(item.id);
+    setInfo("已进入重新修改模式，请上传新的作业图片。");
+    await loadAssignmentDetail(item.id);
+  };
+
+  const cancelEdit = () => {
+    setEditingId(null);
+    setTitle("");
+    clearImages();
+    setInfo("");
+  };
+
   const startCamera = async () => {
     setError("");
     try {
@@ -308,6 +326,7 @@ export default function HomePage() {
     setIsSubmitting(true);
     setInfo("正在提交图片并调用 LLM 批改，请稍候...");
     try {
+      const isEditing = editingId !== null;
       const formData = new FormData();
       formData.append("username", savedUser);
       if (title.trim()) {
@@ -319,8 +338,11 @@ export default function HomePage() {
         formData.append("images", blob, name);
       });
 
-      const res = await fetch(`${apiBase}/assignments`, {
-        method: "POST",
+      const endpoint = isEditing
+        ? `${apiBase}/assignments/${editingId}`
+        : `${apiBase}/assignments`;
+      const res = await fetch(endpoint, {
+        method: isEditing ? "PUT" : "POST",
         body: formData,
       });
       if (!res.ok) {
@@ -328,12 +350,19 @@ export default function HomePage() {
         throw new Error(payload.error || "提交失败");
       }
       const created = (await res.json()) as AssignmentDetail;
-      setAssignments((prev) => [created, ...prev]);
+      if (isEditing) {
+        setAssignments((prev) =>
+          prev.map((item) => (item.id === created.id ? { ...item, ...created } : item))
+        );
+      } else {
+        setAssignments((prev) => [created, ...prev]);
+      }
       setSelectedId(created.id);
       setSelected(created);
       setTitle("");
       clearImages();
-      setInfo("批改完成，可在右侧查看结果。");
+      setEditingId(null);
+      setInfo(isEditing ? "修改完成，已重新批改。" : "批改完成，可在右侧查看结果。");
     } catch (err) {
       setError(err instanceof Error ? err.message : "提交失败");
     } finally {
@@ -354,6 +383,9 @@ export default function HomePage() {
         setSelectedId(null);
         setSelected(null);
       }
+      if (editingId === id) {
+        cancelEdit();
+      }
     } catch (err) {
       setError(err instanceof Error ? err.message : "删除失败");
     }
@@ -414,6 +446,18 @@ export default function HomePage() {
               <span className="tag">IMAGE + LLM</span>
             </header>
             <div className="upload-area">
+              {editingId && (
+                <div className="edit-banner">
+                  <div>
+                    <strong>正在重新修改：</strong>
+                    {selected?.title || title || `作业 #${editingId}`}
+                    <div className="muted">请上传新的作业图片后重新批改。</div>
+                  </div>
+                  <button className="button ghost" onClick={cancelEdit}>
+                    取消修改
+                  </button>
+                </div>
+              )}
               <label className="muted">作业标题</label>
               <input
                 className="input"
@@ -486,7 +530,7 @@ export default function HomePage() {
                 onClick={handleSubmit}
                 disabled={isSubmitting}
               >
-                {isSubmitting ? "批改中..." : "提交批改"}
+                {isSubmitting ? "批改中..." : editingId ? "重新提交批改" : "提交批改"}
               </button>
 
               {error && <div className="feedback">{error}</div>}
@@ -530,6 +574,15 @@ export default function HomePage() {
                         保存图片{imageCountFor(item) > 1 ? ` (${imageCountFor(item)}张)` : ""}
                       </a>
                     )}
+                    <button
+                      className="button secondary"
+                      onClick={(event) => {
+                        event.stopPropagation();
+                        void beginEdit(item);
+                      }}
+                    >
+                      重新修改
+                    </button>
                     <button
                       className="button ghost"
                       onClick={(event) => {