feat: Multi-account support and auth exit fix

[shahariaazam]

Summary

Implements multi-account support allowing users to authenticate and manage multiple Google accounts, plus fixes the auth login hanging issue.

Issues Fixed

✅ Auth login no longer hangs - Command exits immediately after successful authentication
✅ Multi-account support - Manage multiple Google accounts seamlessly

New Features

Multi-Account Management

• Authenticate with multiple Google accounts
• Each account identified by email (auto-detected)
• Set default account for commands
• Switch between accounts with --account flag
• List all authenticated accounts
• Remove specific accounts

New Commands

• gscli auth list - List all authenticated accounts
• gscli auth set-default <email> - Set default account
• gscli auth remove <email> - Remove specific account

Global --account Flag

All commands now support --account <email> option:

• gscli gmail list --account work@company.com
• gscli drive list --account personal@gmail.com
• gscli calendar list --account other@example.com

Usage Examples

# Login with first account
gscli auth login --client ./client.json
# ✅ Authenticated as: user@gmail.com

# Login with second account
gscli auth login --client ./client.json
# (authenticate with different account in browser)
# ✅ Authenticated as: work@company.com

# List all accounts
gscli auth list
# 1. user@gmail.com (default)
# 2. work@company.com

# Set default
gscli auth set-default work@company.com

# Use default account
gscli gmail list

# Use specific account
gscli gmail list --account user@gmail.com
gscli drive list --account work@company.com
gscli calendar list --account personal@example.com

# Remove account
gscli auth remove user@gmail.com

# Logout all
gscli auth logout

Technical Changes

Authentication Flow Fix

• Clear timeout before closing server
• Use server.close(callback) for proper shutdown
• Command exits immediately after success

Multi-Account System

• New accounts.ts module for account management
• Store accounts in ~/.config/gscli/accounts.json
• Added userinfo.email scope to fetch user email
• Email auto-detected via Google OAuth2 API
• Client credentials stored per account

Account Storage

{
  "default_account": "user@gmail.com",
  "accounts": {
    "user@gmail.com": {
      "email": "user@gmail.com",
      "client_id": "...",
      "client_secret": "...",
      "access_token": "...",
      "refresh_token": "...",
      "scope": "...",
      "token_type": "Bearer",
      "expiry_date": 123456789
    }
  }
}

Updated All Commands

• Gmail: list, search, folders-list
• Drive: list, search, download
• Calendar: list, search
  All support --account <email> flag

Testing

• ✅ TypeScript compilation passes
• ✅ Auth exits properly (no hang)
• ✅ Multi-account authentication tested
• ✅ Account switching tested
• ✅ Default account logic tested
• ✅ All commands work with --account flag

Breaking Changes

None - fully backward compatible