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/embedProvider.test.ts

@@ -0,0 +1,22 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { sameProvider } from './embedProvider';
+
+test("sameProvider: a page cannot borrow another provider's embed via og:url (#326)", () => {
+  const yt = 'https://www.youtube.com/watch?v=dQw4w9WgXcQ';
+  assert.equal(sameProvider('https://evil.example/free-gift', yt), false);
+  assert.equal(sameProvider('https://youtube.com.evil.example/x', yt), false);
+  assert.equal(sameProvider('https://youtu.be/abc', yt), true);
+  assert.equal(sameProvider('https://m.youtube.com/shorts/abc', yt), true);
+  assert.equal(
+    sameProvider('https://vm.tiktok.com/ZM123/', 'https://www.tiktok.com/@a/video/1'),
+    true,
+  );
+  assert.equal(sameProvider('https://spotify.link/abc', 'https://open.spotify.com/track/1'), true);
+  assert.equal(sameProvider('https://on.soundcloud.com/x', 'https://soundcloud.com/a/b'), true);
+  assert.equal(
+    sameProvider('https://redd.it/abc', 'https://www.reddit.com/r/x/comments/abc/'),
+    true,
+  );
+  assert.equal(sameProvider('not a url', yt), false);
+});