Fix JSON validation and formatting in scene editor

[mosoriob]

Problem

The 'Validate' and 'Format JSON' buttons in the scene editor (/dataset/potree//edit) are not working properly. Users cannot validate their JSON5 syntax or format their configuration files.

Current Behavior

• Clicking 'Validate' button does not provide feedback on JSON5 syntax
• Clicking 'Format JSON' button does not format/prettify the JSON5 content
• No error messages or success indicators appear

Expected Behavior

• 'Validate' button should check JSON5 syntax and show success/error messages
• 'Format JSON' button should prettify and properly indent the JSON5 content
• Clear user feedback should be provided for both operations

Tasks

• Debug the JavaScript validation function in the edit template
• Fix JSON5 parsing and validation logic
• Ensure format function works with JSON5 syntax (comments, trailing commas)
• Test success and error message display
• Verify proper error handling for invalid JSON5

Files to Check

• src/ckanext-potree/ckanext/potree/templates/potree/edit.html (lines 205-282)
• JavaScript functions: formatBtn and validateBtn event listeners
• JSON5 parsing logic

Acceptance Criteria

• 'Validate' button correctly identifies valid/invalid JSON5 syntax
• 'Format JSON' button properly formats JSON5 content with indentation
• Clear success/error messages appear to the user
• Functions work with JSON5 features (comments, trailing commas, etc.)