Lint shell scripts with ShellCheck in make lint and CI

[regnull]

Goal

Lint the shell scripts in scripts/ with ShellCheck, locally and in CI.

Context

Launching the dev app, starting the packaged backend and publishing releases all go through bash scripts: scripts/electron-dev.sh, scripts/electron-backend.sh and scripts/github-release.sh. make lint and .github/workflows/ci.yml only cover Python and TypeScript.

Acceptance criteria

• make lint runs shellcheck scripts/*.sh when shellcheck is installed, and prints a hint instead of failing when it isn't
• CI runs ShellCheck on every push and pull request (ubuntu-latest ships it)
• Existing findings are fixed, or disabled inline with a one-line reason
• CONTRIBUTING.md mentions the optional ShellCheck install