prd: Markdown Editing Support for TextArea Tag

#8676

## Problem
1. Annotators cannot edit Markdown content directly in Label Studio.
2. They must copy content to external editors, make changes, and paste back—breaking the workflow and risking errors.

## Proof of Concept (PoC)
Working implementation: https://github.com/Zenobia2222/PR-LS/tree/feat/textarea-with-markdown

### Demo


*Watch the demo above to see: syntax highlighting, Edit/Split toggle, real-time stats. ☝️*

### Quick Start
Simply add `markdown="true"` to your TextArea tag:

```xml

</View>
```

## User Stories

**As an annotator**, I want to edit Markdown text directly with syntax highlighting, so I can refine content without switching to external tools.

**As an annotator**, I want to toggle between Edit and Split (preview) modes, so I can verify the rendered output before submitting.

## Acceptance Criteria

- [x] CodeMirror editor with Markdown syntax highlighting
- [x] Edit/Split view mode toggle preserves content
- [x] Real-time character and word count display
- [x] `Shift+Enter` submits (consistent with existing TextArea)
- [x] Works with existing TextArea attributes (`editable`, `perRegion`, `rows`, etc.)
- [x] Backward compatible (defaults to `false`)
- [x] Unit tests + E2E tests (192 lines, ≥80% coverage)
- [x] Documentation updated

## Technical Details

**Implementation**: Reuses existing Label Studio components (CodeMirror via `react-codemirror2`, existing Markdown renderer, MobX state management). No backend changes needed.