Preview pane fails to update automatically when editing Markdown content

### 🐞 Coding Issue: Live Preview not updating

**Description:**
When editing Markdown files in the Decap CMS editor, the preview pane does not refresh automatically after content changes. Users must switch away from the editor or manually trigger a refresh to see updates.

**Steps to Reproduce:**
1. Open Decap CMS (self-hosted or demo version).
2. Create or edit a Markdown-based entry (e.g., blog post).
3. Type some text and wait for the preview to update.
4. Observe that the preview remains stale until focus is lost or a manual save occurs.

**Expected Behavior:**
The preview pane should update automatically as content changes are made in the editor.

**Actual Behavior:**
The preview remains outdated until a manual refresh or navigation change.

**Possible Cause (Hypothesis):**
The `onChange` handler for the Markdown editor may not be properly triggering a state update in the preview component. The issue could be in:
- `decap-cms-core/src/components/Editor/EditorControlPane/EditorControlPane.js`
- or the Redux store not propagating the change to the preview frame.

**Proposed Fix:**
- Ensure that the preview iframe listens for content updates from the editor.
- Add throttled state synchronization to improve performance.
- Validate that `updatePreview()` (or equivalent) fires on every content change event.

**Environment:**
- Decap CMS version: Latest (as of Nov 2025)
- Browser: Firefox 144.0 / Chrome 130+
- Hosting: Local test instance (`npx decap-server`)

**Additional Context:**
This issue affects the user experience when writing Markdown-rich content and can cause confusion for non-technical editors.