fix register validation, review visibility, and search e2e coverage

[tenten-shih]

Summary

• fix register form API error mapping and add missing i18n messages
• sync review and promotion approval state before returning updated tasks
• refine search empty-state behavior and card keyboard interaction
• add e2e coverage for register/login validation and search result interactions

Verification

• bash scripts/run-playwright-with-report.sh e2e --grep "TC_REG_003: shows error when registering with existing username"
• npx playwright test e2e/search-page-full.spec.ts e2e/search-card-interaction.spec.ts --grep "TC_SEARCH_INPUT_003: empty search shows keyword guidance instead of a default list|TC_SEARCH_INTERACT_007: clicking a skill card navigates to the skill detail page"
• ./mvnw -pl skillhub-app -am -Dtest=ReviewServiceTest,PromotionServiceTest,SkillApprovalVisibilityFlowIntegrationTest -Dsurefire.failIfNoSpecifiedTests=false test

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ tenten-shih
❌ huishi3

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