feat(imitation): simplify form fields and make brand_name required

Remove industry, target_audience, content_goal, tone, and length_goal from the imitation form; brand_name is now mandatory with autocomplete from brand library, keywords are pre-populated from the selected brand's search keywords, and output length is hardcoded to ~2000 characters in the prompt.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

apps/admin-web/src/i18n/messages/zh-CN.ts

@@ -691,14 +691,9 @@ const zhCN = {
       sourceURL: "文章 URL",
       sourceTitle: "来源标题",
       language: "文章语言",
-      industry: "行业/场景",
       brandName: "品牌/主体",
       region: "地域",
-      targetAudience: "目标读者",
-      contentGoal: "内容目标",
-      tone: "语气风格",
-      lengthGoal: "篇幅要求",
-      keywords: "关键词",
+      keywords: "优化关键词",
       preservePoints: "必须保留/强调",
       avoidPoints: "需要规避",
       extraRequirements: "其他要求",
@@ -709,19 +704,15 @@ const zhCN = {
       knowledgeBaseHint: "选择后，后台会检索相关知识片段并作为生成参考，未选择则只基于来源文章和当前设置生成。",
       sourceURLPlaceholder: "https://example.com/article",
       sourceTitlePlaceholder: "可选，引用来源标题会自动带入",
-      industryPlaceholder: "例如：家居建材、教育培训、SaaS、医疗健康",
-      brandNamePlaceholder: "可选，填写后文章会自然贴合该品牌或主体",
+      brandNamePlaceholder: "必填，可输入目标达成词，或从品牌库选择",
       regionPlaceholder: "例如：上海、华东、全国、北美市场",
-      targetAudiencePlaceholder: "例如：首次购买用户、渠道代理、企业采购负责人",
-      contentGoalPlaceholder: "例如：科普说明、转化种草、品牌信任、选型建议",
-      tonePlaceholder: "例如：专业可信、轻松口语、评测对比、新闻解读",
-      lengthGoalPlaceholder: "例如：1200字左右、短文、深度长文",
-      keywordsPlaceholder: "输入关键词后回车，可多选",
+      keywordsPlaceholder: "输入优化关键词后回车，可多选；选择品牌库主体后会显示搜索词候选",
       preservePointsPlaceholder: "列出必须保留的事实、数据、观点或案例",
       avoidPointsPlaceholder: "列出不能出现的说法、竞品、合规禁区或表达方式",
       extraRequirementsPlaceholder: "补充行业、渠道、结构、标题风格等自由要求",
       queued: "仿写任务已提交，正在排队生成",
       sourceURLRequired: "请先填写来源文章 URL",
+      brandNameRequired: "请先填写品牌/主体",
       submitError: "仿写任务提交失败",
       backToList: "返回仿写列表",
     },