fix(previews): a page can no longer pose as a YouTube (etc.) player via its og:url (#326)

The embed fallback used the preview's og:url whatever site was posted.
og:url is set by the page, so evil.example could render as a playable
YouTube card (attacker's video) whose links go to evil.example. Only use
an og:url on the same provider as the posted link, including the
provider's own short-link domains (youtu.be, spotify.link, redd.it, …).

Fixes #326

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/components/url-preview/UrlPreviewCard.tsx

@@ -54,6 +54,7 @@ import {
   GIF_PREVIEW_MAX_WIDTH,
   gifPreviewBox,
 } from '../../utils/gifPreviewSize';
+import { sameProvider } from './embedProvider';
 
 const linkStyles = { color: color.Success.Main };
 
@@ -2497,7 +2498,10 @@ export const UrlPreviewCard = as<'div', { url: string; ts: number }>(
     const resolveEmbed = (prev: IPreviewUrlResponse): MediaEmbed | null => {
       if (embed) return embed;
       const ogUrl = prev['og:url'];
-      return typeof ogUrl === 'string' && ogUrl !== url
+      // [Gitea #326] …but only from an og:url on the posted link's own
+      // provider: the page sets og:url, so any site could otherwise show up
+      // as a playable YouTube (etc.) card that links to itself.
+      return typeof ogUrl === 'string' && ogUrl !== url && sameProvider(url, ogUrl)
         ? parseMediaEmbed(ogUrl, window.location.hostname)
         : null;
     };