refactor: 简化评测创建向导规则来源

移除已有维度/基线两种规则来源，评测创建仅保留新建维度流程并自动生成维度名称；维度名称字段回归独立创建页，表单校验改为返回精确布尔值并启用 scroll-to-error。

frontend/src/views/eval/EvalCreateView.vue

@@ -36,10 +36,9 @@ const taskForm = ref<EvalTaskSetupDraft>({
 })
 
 const ruleForm = ref<EvalRuleSetupDraft>({
-  ruleMode: 'existing',
+  ruleMode: 'new',
   dimension_id: '',
   newDimension: {
-    name: '',
     type: '',
     description: '',
     eval_model: '',
@@ -95,7 +94,7 @@ async function loadData() {
 function buildDimensionPayload() {
   const draft = ruleForm.value.newDimension
   const payload: Partial<Dimension> = {
-    name: draft.name,
+    name: `Custom_Dim_${Date.now()}`,
     type: draft.type,
     description: draft.description,
     eval_model: draft.type === 'text_similarity' ? undefined : draft.eval_model,