fix(embeds): quality pass — close button, focus, a11y, perf, SoundCloud revert

From the quality-review agents:
- Revert on.soundcloud.com support: the w.soundcloud widget doesn't follow the
  redirect (needs an oEmbed resolve, deferred).
- Add a Close button to playing video/TikTok embeds and a Collapse button to the
  expanded X post — playback was previously one-way (only escapable by scrolling).
- focus-visible ring on the embed facade (folds resets outline:none, leaving
  keyboard users with no indicator).
- Only subscribe to resize postMessages while the iframe is mounted (was attaching
  a global listener per Instagram/Reddit facade before play).
- TikTok oEmbed fetch now uses AbortController (abort on unmount) + aria-busy /
  'Loading…' label on the resolving spinner.
- Decorative facade thumbnails use alt="" (parent already names them); drop the
  dangling-colon aria-labels when there's no title.
- Cap URL previews at 6 per message so a link-dump can't spawn dozens of fetches.

Tests 728. No CSP change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/url-preview/UrlPreview.css.tsx

@@ -255,6 +255,12 @@ export const EmbedFacade = style([
     ':hover': {
       filter: 'brightness(0.85)',
     },
+    selectors: {
+      '&:focus-visible': {
+        outline: `2px solid ${color.Primary.Main}`,
+        outlineOffset: '-2px',
+      },
+    },
   },
 ]);