Organize login providers into dedicated packages

[briangann]

Summary

Move each login provider into its own package under pkg/kiosk/login/ to isolate them and provide a clear template for adding new providers.

Motivation

With the browser.Browser interface in place (#257), login providers now have clean dependency boundaries:

• Each depends on browser.Browser (interface)
• Each depends on shared config types
• No provider depends on another

This makes a package-per-provider layout natural and useful.

Proposed Structure

pkg/kiosk/login/
  anonymous/
    anonymous.go
    anonymous_test.go
  local/
    local.go
    local_test.go
  gcom/
    gcom.go
  goauth/
    goauth.go
  idtoken/
    idtoken.go
  apikey/
    apikey.go
  aws/
    aws.go
  azuread/
    azuread.go

Shared utilities (GenerateURL, generateExecutorOptions, cycleWindowState, etc.) stay in pkg/kiosk/.

Benefits

• Each provider is isolated and testable independently
• New providers can be created by duplicating an existing directory as a template
• Clear ownership boundaries per login method
• Easier to review and maintain individual providers

Context

• Depends on Extract CDP browser interface to enable login provider unit testing #257 (browser interface extraction) being merged first
• Also a good time to migrate the remaining 6 providers to use browser.Browser
• Shared config types may need to move to a pkg/kiosk/config/ package to avoid circular imports