Plugin CKEditor 5 — CKEditor 5 Plugin

You are a senior CKEditor 5 plugin architect.

I need you to build a complete CKEditor 5 plugin called "NewsletterPlugin".

Context:
- This is a migration from a legacy CKEditor 4 plugin.
- Must follow CKEditor 5 architecture strictly.
- Must use CKEditor 5 UI framework and plugin system.
- Must follow documentation:
  https://ckeditor.com/docs/ckeditor5/latest/framework/architecture/ui-components.html
  https://ckeditor.com/docs/ckeditor5/latest/features/html/general-html-support.html

Environment:
- CKEditor 5 custom build
- ES6 modules
- Typescript preferred (if possible)
- No usage of CKEditor 4 APIs

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FEATURE REQUIREMENTS
========================================

1) Toolbar Button:
- Add a toolbar button named "newsletter"
- Icon: simple SVG placeholder
- When clicked → open a dialog (modal)

2) Dialog Behavior:
The dialog must contain input fields:
- title (text input)
- description (textarea)
- tabs (dynamic list, user can add/remove tab items)
    Each tab item:
        - tabTitle
        - tabContent (HTML allowed)

Buttons:
- Cancel
- OK

3) On OK:
- Generate structured HTML block inside editor
- Structure example:

<div class="newsletter">
    <ul class="newsletter-tabs">
        <li class="active">
            <a href="#tab-1" class="active">Tab 1</a>
        </li>
        <li>
            <a href="#tab-2">Tab 2</a>
        </li>
    </ul>
    <div class="newsletter-content">
        <div id="tab-1" class="tab-pane active">
            Content 1
        </div>
        <div id="tab-2" class="tab-pane">
            Content 2
        </div>
    </div>
</div>

4) Behavior inside editor:

- First tab always active by default.
- When user clicks <a> tab link:
    - Remove class "active" from all tabs and panes
    - Add class "active" to clicked tab and corresponding pane
- When user double-clicks <a>:
    - Open dialog again
    - Load existing data
    - Allow editing
    - Update HTML structure

5) MUST USE:
- GeneralHtmlSupport (GHS) for allowing custom classes & attributes
- Proper upcast / downcast converters
- Widget API (toWidget, toWidgetEditable if needed)
- Command class
- UI Component system (ButtonView, View, InputTextView)
- Editing & UI part separated
- Schema registration properly

6) Architecture required:

Create structure:

- newsletter/
    - newsletterplugin.ts
    - newsletterediting.ts
    - newsletterui.ts
    - newslettercommand.ts

7) Technical requirements:

- Register schema element:
    newsletterBlock
- Must allow:
    class
    id
    href
    data attributes

- Use:
    editor.model.change()
    conversion.for('upcast')
    conversion.for('downcast')

- Handle click event via editing view document
- Use editing.view.document.on( 'click', ... )
- Detect double click event

8) Important:
Do NOT use raw DOM manipulation.
All updates must go through editor.model.

9) Output required:
- Full plugin code
- Proper imports
- Comments explaining architecture
- Explain migration differences from CKEditor 4
- Show how to register plugin in build

10) Extra:
Explain how to enable GeneralHtmlSupport configuration in editor config.

========================================

Please produce clean production-ready code.
Do not simplify logic.
Follow CKEditor 5 best practices strictly.