Exclude rolling Nightly builds from the installers

[LunaStev]

Summary

Exclude the rolling nightly release from automatic installer selection and reject explicit nightly/vnightly requests with a manual-download link. Search subsequent release pages when necessary, while retaining support for ordinary versioned pre-releases.

Motivation

Supports wavefnd/Wave#831. Nightly is intended for direct artifact downloads; publishing it must not disrupt normal compiler installation.

Target and compatibility impact

Updates the Bash and PowerShell installers served by wave-lang.dev. Versioned installation remains supported. This PR must be merged and deployed before the companion Wave Nightly workflow can pass its installer gate; that workflow verifies the reviewed installer contents at both public URLs.

Validation

• bash frontend/tests/install_checksum_test.sh
• bash frontend/tests/install_nightly_test.sh
• pwsh -NoProfile -File frontend/tests/install_nightly_test.ps1 (PowerShell 7.6.6 on Linux)
• Bash syntax checks, Actionlint, and git diff --check.
• Regressions cover explicit Nightly rejection, automatic selection, pagination, empty responses, and request failures. Installer tests are added to frontend CI. No live installation or deployment was performed.

Checklist

• Commits include a DCO Signed-off-by line.
• Tests cover new behavior.
• Rejected Nightly requests include an actionable diagnostic.