Expand direct image links instead of showing nothing (#43 follow-up)

A URL that points straight at an image file (Content-Type: image/*) has
no HTML to scrape Open Graph tags from, so the fetch found nothing and
the message showed no preview at all -- reported against
https://imgs.xkcd.com/comics/creepy.png.

link_preview_service now recognizes an allowed image content-type (same
list storage.py uses for uploads) before falling through to the HTML/og:
path, and returns the URL itself as the preview (LinkPreview.is_image).
No need to download the body -- the already-SSRF-validated URL is the
image. The frontend renders that case as a real expandable image
(message-image + lightbox, same as an actual attachment) instead of the
small title+description card, which would have nothing to show anyway.

Verified end-to-end against the reported URL.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/ChatPane.tsx

@@ -158,6 +158,7 @@ export function ChatPane({
             description: envelope.description,
             image_url: envelope.image_url,
             site_name: envelope.site_name,
+            is_image: envelope.is_image,
           }
           setHistory((prev) => prev.map((m) => (m.id === envelope.id ? { ...m, link_preview: linkPreview } : m)))
           setLive((prev) => prev.map((m) => (m.id === envelope.id ? { ...m, link_preview: linkPreview } : m)))