chore: fix some annoying Bash quirks

[mdqst]

This PR:

• Fixes a few Bash annoyances that were breaking the script sometimes:
  • --account-index check now properly handles empty vs number, older Bash hated the old version.
  • Docker flag check fixed (if [ "$docker" = true ]; then) so it doesn’t try to run “true” as a command.
  • Commands with spaces now work because cmd is an array.
  • Added a quick command -v cast check so the script won’t crash if cast isn’t installed.
  • Running commands now uses $("${cmd[@]}" ...) to handle spaces properly.

This PR does not:

• Touch any features, just fixes how the script behaves in different Bash setups.
• Change anything unrelated.

Key places to review:

• Account index parsing
• Docker flag handling
• Anywhere cmd is used to run commands
• Cast command usage