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/en-US.ts

@@ -723,14 +723,9 @@ const enUS = {
       sourceURL: 'Article URL',
       sourceTitle: 'Source title',
       language: 'Article language',
-      industry: 'Industry / scenario',
       brandName: 'Brand / subject',
       region: 'Region',
-      targetAudience: 'Target audience',
-      contentGoal: 'Content goal',
-      tone: 'Tone',
-      lengthGoal: 'Length',
-      keywords: 'Keywords',
+      keywords: 'Optimization keywords',
       preservePoints: 'Keep / emphasize',
       avoidPoints: 'Avoid',
       extraRequirements: 'Extra requirements',
@@ -743,21 +738,17 @@ const enUS = {
         'Selected groups are retrieved in the backend and passed to generation as reference context.',
       sourceURLPlaceholder: 'https://example.com/article',
       sourceTitlePlaceholder: 'Optional. Citation titles are prefilled automatically.',
-      industryPlaceholder: 'For example: home improvement, education, SaaS, healthcare',
-      brandNamePlaceholder: 'Optional. Helps the article fit a specific brand or subject.',
+      brandNamePlaceholder: 'Required. Type the target subject, or choose from the brand library.',
       regionPlaceholder: 'For example: Shanghai, East China, nationwide, North America',
-      targetAudiencePlaceholder:
-        'For example: first-time buyers, channel partners, procurement leads',
-      contentGoalPlaceholder: 'For example: education, conversion, brand trust, selection advice',
-      tonePlaceholder: 'For example: professional, conversational, comparative, news analysis',
-      lengthGoalPlaceholder: 'For example: around 1200 words, short post, long-form guide',
-      keywordsPlaceholder: 'Type keywords and press Enter',
+      keywordsPlaceholder:
+        'Type optimization keywords and press Enter. Brand-library options appear after choosing a subject.',
       preservePointsPlaceholder: 'List facts, data, arguments, or examples that must be kept.',
       avoidPointsPlaceholder: 'List claims, competitors, compliance risks, or wording to avoid.',
       extraRequirementsPlaceholder:
         'Add channel, structure, title style, or industry-specific instructions.',
       queued: 'Rewrite task submitted and queued.',
       sourceURLRequired: 'Enter the source article URL first.',
+      brandNameRequired: 'Enter the brand / subject first.',
       submitError: 'Failed to submit rewrite task.',
       backToList: 'Back to rewrites',
     },