fix(commands): show validation errors in modal instead of globally

[JiahaoZhu11]

Summary

• Display command validation errors directly in the modal footer instead of using the global error banner
• Improves UX by keeping error context visible while user corrects the issue

Changes

• Add commandModalError signal for modal-specific error state
• Add error prop to CommandModal component
• Update saveCommand to use modal-specific error for validation failures
• Clear modal error when modal is closed
• Display error message in modal footer (left side, buttons on right)

Test plan

• Try to save command with empty name → error appears in modal footer
• Try to save command with empty template → error appears in modal footer
• Close modal → error clears
• Fix validation issue and save → command saves successfully
• Verify global error banner is not triggered for validation errors

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[JiahaoZhu11]

Hey @benjaminshafii, just realized I forgot to mark this one as "ready for review" earlier. This is another quick fix—would appreciate it when you have a moment to take a look. Thanks!