Block: Refactor frontend to use Interactivity API

[michakrapp]

Refactor the docs-viewer block's frontend to leverage the WordPress Interactivity API for improved performance and user experience.

Goals:

• Lazy-load file content instead of rendering on page load
• Smooth transitions when switching files
• Better performance with deferred rendering
• Use Interactivity API directives (wp-interactive, wp-context, wp-bind, etc.)

Benefits:

• Smaller initial page payload
• Faster Time to Interactive
• More interactive/animated user experience

Acceptance Criteria:

• Block uses Interactivity API directives
• Content loads lazily (deferred)
• Smooth transitions between states
• Graceful handling of loading/error states