Support background image via slide front matter

## Summary

There is currently no way to set an image as a slide background from the markdown file. Users writing slides in markdown cannot specify a background image that renders full-bleed behind the slide content.

## Current Behavior

- `SlideOptions` only supports `title`, `style`, and `template` as known front matter fields.
- The `BackgroundPart` in the superdeck package renders an empty `SizedBox.shrink()`.
- `SlideView` supports a `backgroundWidget` via `SlideParts`, but this is a programmatic Dart API — not configurable from markdown.
- The `@image` block is a content block that takes up layout space, so it cannot be used as a background.

## Expected Behavior

Users should be able to specify a background image directly in markdown front matter, for example:

```markdown
---
title: My Slide
background: assets/hero-image.jpg
---

# Welcome
```

The image should render full-bleed behind all slide content (sections, header, footer), supporting common `BoxFit` options (e.g., `cover`, `contain`).

## Proposed Approach

1. Add `background` as a known field in `SlideOptions` (in `core`).
2. When a slide has a `background` value, the rendering layer should resolve it as an image source (asset path, file path, or URL) and render it as a positioned background behind the slide content.
3. Consider supporting an extended syntax for fit and alignment:

```markdown
---
background:
src: assets/hero-image.jpg
fit: cover
alignment: center
opacity: 0.5
---
```

## Additional Context

- The test fixture `complete_config.yaml` already has a `background: background.jpg` entry, suggesting this was anticipated but never implemented.
- The `SlideParts.background` widget slot already exists in `SlideView`, so the rendering plumbing is partially in place — it just needs a markdown-driven path.