fix: 固定开发环境后端数据目录，避免 cwd 不同导致头像 404

从 cwd 向上解析 backend 根，并写入本地开发启动约定。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/config/config_test.go

@@ -0,0 +1,90 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func backendRoot(t *testing.T) string {
+	t.Helper()
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	root, err := filepath.Abs(filepath.Join(wd, ".."))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return root
+}
+
+func assertPath(t *testing.T, got, want string) {
+	t.Helper()
+	g, err := filepath.Abs(got)
+	if err != nil {
+		t.Fatal(err)
+	}
+	w, err := filepath.Abs(want)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if filepath.Clean(g) != filepath.Clean(w) {
+		t.Fatalf("路径不符:\n  got  %s\n  want %s", g, w)
+	}
+}
+
+func TestResolveWorkPathFromOfficialCwd(t *testing.T) {
+	root := backendRoot(t)
+	t.Chdir(root)
+	got, err := resolveWorkPath()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertPath(t, got, root)
+}
+
+func TestResolveWorkPathFromCmdJiang13(t *testing.T) {
+	root := backendRoot(t)
+	t.Chdir(filepath.Join(root, "cmd", "jiang13"))
+	got, err := resolveWorkPath()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertPath(t, got, root)
+}
+
+func TestResolveWorkPathFromRepoRoot(t *testing.T) {
+	root := backendRoot(t)
+	t.Chdir(filepath.Join(root, ".."))
+	got, err := resolveWorkPath()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertPath(t, got, root)
+}
+
+func TestResolveWorkPathEnvOverride(t *testing.T) {
+	root := backendRoot(t)
+	t.Setenv("JIANG13_WORK_PATH", root)
+	t.Chdir(t.TempDir())
+	got, err := resolveWorkPath()
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertPath(t, got, root)
+}
+
+func TestRejectStrayDataDir(t *testing.T) {
+	err := rejectStrayDataDir(`C:\proj\backend\cmd\jiang13\data`)
+	if err == nil {
+		t.Fatal("期望拒绝 cmd/jiang13/data")
+	}
+	if !strings.Contains(err.Error(), "拒绝使用数据目录") {
+		t.Fatalf("错误文案不符: %v", err)
+	}
+	if err := rejectStrayDataDir(`C:\proj\backend\data`); err != nil {
+		t.Fatal(err)
+	}
+}