Add python3 prerequisite check to pre-build-check.sh

[Zena-park]

Context

Flagged in PR #35 code review (comment).

Problem

crates/desktop-app/ui/scripts/pre-build-check.sh uses python3 for JSON validation but doesn't check it in prerequisites. This can cause confusing failures on systems without Python installed.

Proposed Fix

1. Add a command -v python3 prerequisite check at the top of the script
2. Or switch to a Node-based JSON validation since Node is already a required dependency