Allow previewable PDFs in image galleries

Galleries currently accept raster images only:

```js
// src/nodes/image_gallery_node.js
static isValidChild(node) {
return $isActionTextAttachmentNode(node) && node.isPreviewableImage
}

// src/helpers/html_helper.js
function isPreviewableImage(contentType) {
return contentType.startsWith("image/") && !contentType.includes("svg")
}
```

Because that is a content-type test rather than a "does this have a preview" test, it excludes PDFs even when Active Storage has generated a thumbnail and Lexxy is already displaying it.

That feels inconsistent with how Lexxy treats a previewable PDF everywhere else. `ActionTextAttachmentNode` renders it down the previewable path with `previewSrc` and `pendingPreview`, `BlobWithPreviewUrl` supplies a `resize_to_limit: [1024, 768]` preview URL, and the figure is given `attachment--preview`. A gallery is a layout container, and a PDF that already has a thumbnail lays out much like an image does.

Concretely: three scanned pages attached as PDFs cannot be arranged as a three-up gallery, while the same three pages exported as PNGs can.

## The obvious change, and why it may not be enough

The accessor already exists:

```js
get isPreviewableAttachment() {
return this.isPreviewableImage || this.previewable
}
```

Swapping `isValidChild` to use it would admit previewable PDFs while still excluding plain file attachments, which have no thumbnail and would not lay out sensibly. But it raises a few questions I do not think have obvious answers:

- **Videos come along for the ride.** `isPreviewableAttachment` is `isPreviewableImage || previewable`, so a `` with controls would become gallery-eligible too. That may be desirable, or may want its own predicate.
- **Aspect ratio.** `.attachment-gallery .attachment` is a third-width cell. An A4 page at roughly 3:4 in a third of the post width ends up small and very tall beside landscape photos. Galleries may need a max height, or a per-item aspect hint.
- **Captions.** Host apps render their own `active_storage/blobs/_blob` partial and receive `in_gallery: true`. Anything that puts a filename, size or download link under a standalone PDF would need to suppress it inside a gallery, or every cell gains a line of metadata.
- **Consistency across entry points.** Upload grouping and `#pastedImageFilesFrom` count images to decide whether to form a gallery, so those would want the same predicate. Otherwise uploading three PDFs at once behaves differently from dragging three together.

Happy to have a go at a PR if there is an appetite for it and you have a view on the video question and the aspect-ratio handling. If you would rather galleries stayed images-only, that is a reasonable answer too, in which case the drop affordance is the thing worth tightening: see #1235, where the drop markers appear over targets that cannot accept the drop.

## Version

lexxy 0.9.29, Rails 8.2.0.alpha, Active Storage with the Poppler previewer.

Start with src/nodes/image_gallery_node.js and src/helpers/html_helper.js, then trace ActionTextAttachmentNode, BlobWithPreviewUrl, and the upload-grouping and #pastedImageFilesFrom entry points. Review the .attachment-gallery styling and host active_storage/blobs/_blob partial behavior. Done requires a settled policy for PDFs, videos, aspect ratios, captions, and consistent gallery formation across entry points.

Written by the indexing model from the issue text.