fix(brand-questions): surface seed topic min length as actionable error

Reject AI seed topics shorter than 2 characters on both client and server with
a dedicated invalid_seed_topic error code, and render an inline hint listing
the offending terms so users know exactly what to fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/i18n/messages/en-US.ts

@@ -1519,6 +1519,10 @@ const enUS = {
       },
       errors: {
         emptyInput: 'Enter search term content first.',
+        aiSeedTopicTooShort:
+          '{terms} is too short. Enter a user search term with at least 2 characters, such as "mattress recommendation" or "how to choose a kids bed".',
+        aiSeedTopicTooShortWithMore:
+          '{terms} and {count} more search terms are too short. Enter user search terms with at least 2 characters, such as "mattress recommendation" or "how to choose a kids bed".',
         noSelection: 'Select at least one candidate.',
         noValidQuestions: 'No valid search terms can be saved. Adjust candidates and try again.',
       },