Fix clean CI and Pages deployment setup

[lurui1997]

Summary

• generate the ignored gallery index before Vitest runs on a clean checkout
• isolate gallery rendering tests from deployment environment variables
• keep Pages configuration in the privileged deploy job while requiring repository-level Pages setup

Verification

• pnpm check (232 tests, zero Astro diagnostics)
• exact GitHub Actions environment reproduced locally
• PR and non-default branch jobs remain read-only and deployment-free