Add fullscreen playground interface for entity-context workflows

Summary

Implement a dedicated fullscreen playground UI that removes Drupal's admin chrome (toolbar, navigation, breadcrumbs) to provide an immersive, distraction-free experience when running entity-context workflows. This creates a focused environment ideal for content validation and AI-assisted content review.

Business Value

• Provides distraction-free environment for focused workflow interactions
• Creates professional presentation suitable for content editors and reviewers
• Improves usability by maximizing screen real estate for the playground interface
• Enables embedding playground in custom interfaces without admin toolbar interference

Features Implemented

• Fullscreen HTML Template: Minimal HTML structure without admin modules
• Clean Page Template: Removes breadcrumbs, local actions, and secondary toolbars
• Dynamic Page Titles: Shows workflow name and entity label for context
• Auto-Run Configuration: Configurable based on workflow type
• Chat Input Visibility: Conditional display based on workflow requirements

Technical Implementation

• EntityContextPlaygroundController for UI rendering and configuration
• Route: /admin/flowdrop/workflows/{workflow_id}/playground/entity
• Query parameters: entity_type, entity_id, revision_id, bundle, session_name
• Custom theme suggestions via hook_theme_suggestions_html_alter() and hook_theme_suggestions_page_alter()
• Twig templates: html--flowdrop-entity-playground-fullscreen.html.twig and page--flowdrop-entity-playground.html.twig
• Entity context info passed to frontend JavaScript via drupalSettings

URL Access Pattern

/admin/flowdrop/workflows/content_validator/playground/entity?entity_type=node&entity_id=123&bundle=article