feat: add project-scoped variable support

[fank]

Summary

• Adds --project flag to all variable subcommands (list, get, create, update, delete) to support n8n's project-scoped variables (GA since n8n v1.118.0)
• Adds ProjectID field to Variable struct and threads projectId through ListVariables and CreateVariable API methods
• variable list dynamically shows a PROJECT column when any returned variable is project-scoped

Usage

# Global variables (unchanged)
n8nctl variable list
n8nctl variable create api_url https://api.example.com

# Project-scoped variables
n8nctl variable list --project pJK8xB2nFqWdR0yT
n8nctl variable create env production --project pJK8xB2nFqWdR0yT
n8nctl variable get env --project pJK8xB2nFqWdR0yT
n8nctl variable update env staging --project pJK8xB2nFqWdR0yT
n8nctl variable delete env --project pJK8xB2nFqWdR0yT

Test plan

• go build ./... compiles cleanly
• go vet ./... passes
• n8nctl variable list --help shows --project flag
• n8nctl variable create --help shows --project flag
• n8nctl variable list still works without --project (backward compatible)
• n8nctl variable create key val --project <id> creates a project-scoped variable
• n8nctl variable list --project <id> filters to that project's variables

[gemini-code-assist]

Code Review

This pull request introduces project-scoping for variables by updating the API client and adding a "--project" flag to all variable CLI subcommands. The "list" command now conditionally displays a project column when project-scoped variables are present. Review feedback suggests improving the API client's consistency by returning a "*ListResult" with pagination support, refactoring the CLI table rendering to reduce duplication, and defining the "--project" flag as a persistent flag on the parent command to minimize boilerplate.