test: validate static files in standalone build

[elitan]

Problem

test-install.yml doesn't verify that /_next/static/ files load correctly via HTTP. This is a common tarball packaging issue.

Proposed solution

Add HTTP request to verify static files are served:

• Request a known static file path
• Verify 200 response
• Could extract a path from .next/static/ and request it

Current coverage

• test-install.yml: checks ls .next/static/ exists but doesn't test HTTP serving
• release.yml: now checks files exist but doesn't test HTTP serving