fix(ci): allow manual website rebuild trigger and validate secret

[therealklanni]

Summary

Fix the website rebuild GitHub Actions workflow.

Changes

• Add workflow_dispatch for manual runs from the Actions UI.
• Keep automatic runs on released GitHub Releases.
• Remove the invalid job-level secret condition.
• Validate CF_PAGES_BUILD_HOOK_URL in a step and fail clearly when it is missing.
• Keep the existing Cloudflare build-hook retry behavior.

Verification

• YAML parsed successfully.
• Confirmed both workflow_dispatch and release triggers are present.
• Confirmed no job-level secret if remains.
• Lefthook fmt, clippy, and commit verification passed.
• Branch created from current origin/main at dcf2b7c.