feat(skill): add namespace search and bundle download

[dongmucat]

Summary

• Support namespace search parsing with @namespace and preserve namespace state in the search URL.
• Add namespace filtering UI on the search page and namespace-page actions for download all, selected download, and copy install manifest.
• Add namespace bundle download API with optional selected skill filtering, i18n errors, and download rate limiting.

Verification

• git diff --check origin/main...HEAD
• ./mvnw -pl skillhub-app,skillhub-domain -am -Dtest=SkillControllerDownloadTest,SkillDownloadServiceTest -Dsurefire.failIfNoSpecifiedTests=false test
• pnpm exec vitest run src/shared/lib/search-query.test.ts src/pages/search.test.tsx src/pages/namespace.test.tsx
• pnpm exec tsc --noEmit

Notes

• make generate-api and smoke E2E were previously blocked locally because the backend could not start without PostgreSQL on localhost:5432; CI should provide the authoritative environment-backed signal.

Refs #478