Add interactive prompts for Xero credentials during auth login

[paulmeller]

Summary

Enhanced the auth login command to interactively prompt users for missing Xero credentials (Client ID and Client Secret) when running in a TTY environment, with an option to save them to the config file.

Key Changes

• Interactive credential prompts: When Client ID is missing and the command is running in a TTY (not in --no-prompt mode), users are now prompted to enter their credentials instead of receiving an error
• New prompt utility functions: Added three new helper functions to cmdutil/flags.go:
  • PromptString(): Prompts for regular text input
  • PromptSecret(): Prompts for masked password input using golang.org/x/term
  • PromptConfirmDefault(): Prompts for yes/no confirmation with "yes" as default
• Config file persistence: After collecting credentials via prompts, users are offered the option to save them to the config file for future use
• Smart save detection: Only prompts to save if credentials were actually entered via prompts (not already present in environment variables or config file)

Implementation Details

• Respects the --no-prompt flag to disable interactive prompts in non-TTY environments
• Checks both file-based and environment variable sources before prompting
• Provides user-friendly error messages and feedback throughout the flow
• Gracefully handles save failures with a warning rather than aborting the login process

https://claude.ai/code/session_01De6cUAbiu4rH33XRihzhoa