feat(admin): 新增 CRS 同步预览和账号选择功能

- 后端新增 PreviewFromCRS 接口，允许用户先预览 CRS 中的账号
- 后端支持在同步时选择特定账号，不选中的账号将被跳过
- 前端重构 SyncFromCrsModal 为三步向导：输入凭据 → 预览账号 → 执行同步
- 改进表单无障碍性：添加 for/id 关联和 required 属性
- 修复 Back 按钮返回时的状态清理
- 新增 buildSelectedSet 和 shouldCreateAccount 的单元测试
- 完整的向后兼容性：旧客户端不发送 selected_account_ids 时行为不变

backend/internal/service/crs_sync_helpers_test.go

@@ -0,0 +1,112 @@
+package service
+
+import (
+	"testing"
+)
+
+func TestBuildSelectedSet(t *testing.T) {
+	tests := []struct {
+		name     string
+		ids      []string
+		wantNil  bool
+		wantSize int
+	}{
+		{
+			name:    "nil input returns nil (backward compatible: create all)",
+			ids:     nil,
+			wantNil: true,
+		},
+		{
+			name:     "empty slice returns empty map (create none)",
+			ids:      []string{},
+			wantNil:  false,
+			wantSize: 0,
+		},
+		{
+			name:     "single ID",
+			ids:      []string{"abc-123"},
+			wantNil:  false,
+			wantSize: 1,
+		},
+		{
+			name:     "multiple IDs",
+			ids:      []string{"a", "b", "c"},
+			wantNil:  false,
+			wantSize: 3,
+		},
+		{
+			name:     "duplicate IDs are deduplicated",
+			ids:      []string{"a", "a", "b"},
+			wantNil:  false,
+			wantSize: 2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := buildSelectedSet(tt.ids)
+			if tt.wantNil {
+				if got != nil {
+					t.Errorf("buildSelectedSet(%v) = %v, want nil", tt.ids, got)
+				}
+				return
+			}
+			if got == nil {
+				t.Fatalf("buildSelectedSet(%v) = nil, want non-nil map", tt.ids)
+			}
+			if len(got) != tt.wantSize {
+				t.Errorf("buildSelectedSet(%v) has %d entries, want %d", tt.ids, len(got), tt.wantSize)
+			}
+			// Verify all unique IDs are present
+			for _, id := range tt.ids {
+				if _, ok := got[id]; !ok {
+					t.Errorf("buildSelectedSet(%v) missing key %q", tt.ids, id)
+				}
+			}
+		})
+	}
+}
+
+func TestShouldCreateAccount(t *testing.T) {
+	tests := []struct {
+		name        string
+		crsID       string
+		selectedSet map[string]struct{}
+		want        bool
+	}{
+		{
+			name:        "nil set allows all (backward compatible)",
+			crsID:       "any-id",
+			selectedSet: nil,
+			want:        true,
+		},
+		{
+			name:        "empty set blocks all",
+			crsID:       "any-id",
+			selectedSet: map[string]struct{}{},
+			want:        false,
+		},
+		{
+			name:        "ID in set is allowed",
+			crsID:       "abc-123",
+			selectedSet: map[string]struct{}{"abc-123": {}, "def-456": {}},
+			want:        true,
+		},
+		{
+			name:        "ID not in set is blocked",
+			crsID:       "xyz-789",
+			selectedSet: map[string]struct{}{"abc-123": {}, "def-456": {}},
+			want:        false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := shouldCreateAccount(tt.crsID, tt.selectedSet)
+			if got != tt.want {
+				t.Errorf("shouldCreateAccount(%q, %v) = %v, want %v",
+					tt.crsID, tt.selectedSet, got, tt.want)
+			}
+		})
+	}
+}