feat(RepeaterTable): 컬럼 너비 자동 맞춤 기능 추가

- 균등 분배 / 자동 맞춤 토글 방식으로 변경
- measureTextWidth(): 한글/영문/숫자별 픽셀 계산
- applyAutoFitWidths(): 글자 너비 기준 컬럼 조정
- 계산 규칙 결과 필드를 드롭다운으로 변경

frontend/lib/registry/components/universal-form-modal/modals/TableSectionSettingsModal.tsx

@@ -1232,12 +1232,27 @@ export function TableSectionSettingsModal({
 
                     {(tableConfig.calculations || []).map((calc, index) => (
                       <div key={index} className="flex items-center gap-2 border rounded-lg p-2 bg-muted/30">
-                        <Input
-                          value={calc.resultField}
-                          onChange={(e) => updateCalculation(index, { resultField: e.target.value })}
-                          placeholder="결과 필드"
-                          className="h-8 text-xs w-[150px]"
-                        />
+                        <Select
+                          value={calc.resultField || ""}
+                          onValueChange={(value) => updateCalculation(index, { resultField: value })}
+                        >
+                          <SelectTrigger className="h-8 text-xs w-[150px]">
+                            <SelectValue placeholder="결과 필드 선택" />
+                          </SelectTrigger>
+                          <SelectContent>
+                            {(tableConfig.columns || []).length === 0 ? (
+                              <SelectItem value="__no_columns__" disabled>
+                                컬럼 설정에서 먼저 컬럼을 추가하세요
+                              </SelectItem>
+                            ) : (
+                              (tableConfig.columns || []).map((col) => (
+                                <SelectItem key={col.field} value={col.field}>
+                                  {col.label || col.field}
+                                </SelectItem>
+                              ))
+                            )}
+                          </SelectContent>
+                        </Select>
                         <span className="text-xs text-muted-foreground">=</span>
                         <Input
                           value={calc.formula}