Add orphan cleanup admin settings form

Problem/Motivation

The orphan cleanup functionality currently relies on configuration values that cannot be managed through the Drupal administrative interface.

Site administrators should be able to configure orphan cleanup behavior without manually editing configuration files or using Drush commands.

Steps to reproduce

1. Enable the Block Content Explorer module
2. Attempt to configure orphan cleanup settings from the Drupal admin UI
3. Observe that there is no settings form for managing cleanup configuration

Proposed resolution

• Create src/Form/SettingsForm.php
• Extend ConfigFormBase
• Add configuration form route for:
  

  admin/config/content/block-explorer/settings

• Add form fields for:
  • Enable automatic cleanup
  • Cleanup interval (days)
  • Maximum blocks processed per cron run
• Use Drupal #states to hide cleanup fields when auto-cleanup is disabled
• Save configuration values to block_content_explorer.settings

Remaining tasks

• Create settings form class
• Add configuration form route
• Add menu link for admin configuration page
• Implement form validation and submission handling
• Add conditional field visibility using #states
• Test configuration save functionality

User interface changes

Adds a new admin configuration page:

admin/config/content/block-explorer/settings

The page provides configurable orphan cleanup settings for site administrators.

API changes

None.

Data model changes

None.