v3 email editor cannot open legacy v2 "Classic"/HTML templates

# [Bug] v3 email editor cannot open legacy v2 "Classic"/HTML templates (`_edit_type=html`) — "invalid block" and white-text-on-lost-background broken render

## Environment
- FluentCRM: 3.1.0
- FluentCRM Pro: 3.1.0
- WordPress: 7.0
- WooCommerce: 10.8.1
- PHP: 8.3.30
- Database: MariaDB 11.4.10
- Theme: Astra child
- Note: templates were created in 2.9.x (where they work); the editor breaks them after upgrading to 3.1.0. We had to downgrade to 2.9.87 / Pro 2.9.86 so the client could keep building newsletters.

## Summary

Email templates created with the v2.9.x classic / HTML editor are stored as `fc_template` posts whose `post_content` is raw, table-based HTML with inline styles, and meta `_edit_type = 'html'` (in our case also `_design_template = 'simple'`).

When such a template is opened in the v3.0+ block-based (Vue 3) editor:

1. The editor shows the toast "Invalid blocks were auto-recovered and saved" and tries to "recover" the content by dumping the raw HTML into text blocks, and

2. in doing so it drops the table-cell `background-color` styling. Text that was intentionally white (`color:#ffffff`) on a colored cell then sits on a white/default background → all text becomes invisible (appears white) and the layout collapses.

Where it happens: the server side passes the legacy template through faithfully — `TemplateController::template()` returns `'edit_type' => 'html'` together with the raw `post_content` (FluentCRM 3.1.0, `app/Http/Controllers/TemplateController.php` ~L48–83). The breakage is in the v3 client-side block editor (`assets/BlockComposer.js`), which parses the `edit_type=html` content into blocks, marks them invalid, and "auto-recovers" them lossily instead of rendering the HTML faithfully.

## Reproduction

1. In v2.9.x, create an email template with the Classic/HTML editor (`_edit_type='html'`) containing white text on a colored table cell, e.g.:

```html

\
  \\
    \
      \Newsletter title\
    \
  \\
\
```

1. Upgrade to 3.1.0.

2. Email Templates → open the template in the editor.

3. Observe the "invalid block" error and/or the colored backgrounds gone, leaving white text invisible and the table layout broken.

Our real template had 25 `color:#ffffff` spans on `#fcae23` / `#708cb1` cells — every one of them became invisible.

## Impact

Existing customers with v2.x classic/HTML newsletter templates cannot edit or use them after upgrading to v3.x. For a business that sends a weekly newsletter from a fixed branded template, this is a blocking regression — it forced us to roll back to 2.9.x.

## Expected

The v3 editor should either:

- render legacy `_edit_type=html` templates faithfully in a compatible "classic / raw HTML" mode (preserving raw HTML, inline styles, and especially table-cell `background-color`), or

- provide a lossless one-time migration.

It must not silently drop `background-color` (which inverts white-on-color text into invisible white-on-white), and it should not reject legacy templates as "invalid block" without a fallback.

## Attachments

- Offending template export (raw HTML, `_edit_type=html`, `_design_template=simple`).

- Screenshot 1: the "Invalid blocks were auto-recovered and saved" toast + the collapsed/blank render (white text invisible).

- Screenshot 2: the raw HTML dumped into text blocks (the parser could not interpret the classic HTML).

Thanks!