Standardize and harden Arch Linux setup scripts

[davgar99]

What changed

• enable set -euo pipefail across all nine Bash scripts in the repository for consistent fail-fast behavior
• add the CachyOS-recommended 12 GB Mesa shader cache setting for AMD
• keep the CachyOS-recommended 12 GB NVIDIA shader cache setting and store both cache overrides in ~/.config/environment.d/gaming.conf
• update existing shader cache entries instead of blindly duplicating them
• leave Intel shader cache configuration alone because the current CachyOS Gaming guide does not document an Intel-specific override
• use systemctl enable --force for GDM and SDDM so an existing display-manager.service symlink can be replaced as the scripts already indicate
• standardize NVIDIA capitalization in the NVIDIA setup scripts

Why

These changes consolidate the earlier consistency and failure-handling work into one PR. The repository's Bash scripts now use the same strict shell behavior, while GPU-specific tuning remains limited to settings actually documented by CachyOS rather than inventing equivalent tweaks for every vendor.

The display-manager change also matches the scripts' existing behavior/message when switching between GDM and SDDM.

Scope

This PR supersedes PR #1 and PR #2 so the repository only needs one review for this set of related setup-script improvements.

Validation

• confirmed the branch changes exactly the nine Bash scripts currently found in the repository
• confirmed Intel receives no invented shader-cache override
• CachyOS guidance is used for the AMD/NVIDIA 12 GB shader cache settings

Hardware-specific package installation and privileged system changes should still be exercised on the relevant Arch Linux systems before merging.