Remove dead pve-modal selectors from the global stylesheet #8925

Task Summary

The global stylesheet frontend/src/styles.scss has two selectors in its Python-environment modal (.pve-modal) block that match no element:

• .pve-modal .system-header .subtitle: the only .system-header element holds just a .title.
• The bare .package-inputs token in .package-header-row, .package-inputs: the templates use system-package-inputs and user-package-inputs, which are different class tokens.

Introduced by #4484 (2026-05-01, "feat: Add Python Virtual Environment Support: Creating a Virtual Environment"), which added the whole .pve-modal block.

Usage removed by:

• .subtitle: never used. No .subtitle element has ever existed under .system-header.
• .package-inputs: feat: Add Python Virtual Environment Support: Installing User Defined Packages #4902 (2026-05-12, "Installing User Defined Packages") renamed its element to system-package-inputs.

This is a pure deletion with no behaviour change.

Task Type

• Refactor / Cleanup