[Script request] Add posts cover Front-Matter for themes

**Theme Branch**:

- [x] Aurora：cover
- [x] Fluid：index_img 95961a4
- [ ] Stun：top_image
- [x] Icarus：cover
- [x] Keep：V/A
- [ ] Kaze： banner_img
- [ ] Inside：thumbnail
- [x] Stellar: cover
- [x] Minidyne: V/A
- [x] Landscape: V/A

**Describe the solution you'd like**
Change `cover: ` in the posts front-matter to the themes required field, for example, theme **Fluid** uses `index_img`, so we need this script.

```js
// scripts/meta-space-post-filter.js
hexo.extend.filter.register('before_post_render', function (data) {
data.cover && (data.index_img = data.cover);
return data;
});
```

**Additional context**:

If you're resolving one of the pending requests, please comment and link to this issue in your PR, so I can mark it as resolved.

Start with the requested script entry point, scripts/meta-space-post-filter.js, and its before_post_render filter. Review the listed theme front-matter mappings, including Stun's top_image, Kaze's banner_img, and Inside's thumbnail. Done means the script maps cover to each theme's required field and the pending requests can be marked resolved through linked PRs.

Written by the indexing model from the issue text.