Add strict error handling to all scripts:
- 13 scripts get `set -euo pipefail`
- install-macos-defaults.sh gets `set -uo pipefail` (without -e) because
defaults write commands may fail on newer macOS versions
- install-cargo-packages.sh: also add missing source of shared.sh
Critical bugs where error paths print a message but don't stop execution:
- install-fonts.sh: cd failure now exits properly
- install-ntfy.sh: unsupported OS case now exits with error
- install-git-crypt.sh: git clone and cd failures now exit properly
