feat(embeds): fallback for hung embeds and deleted X posts (#200)

Probed how real providers fail before picking signals:
- X renders an EMPTY frame for a deleted/private/suspended post and says
  so only via postMessage `twttr.private.no_results`. The post embed now
  swaps to "This post isn't available…" with an "Open on X" link.
- A hung frame never fires `load`. After 20 s every player (media, rich
  posts, TikTok, Steam widget, X) overlays "This embed is taking too long
  to load" with Retry (remounts the iframe) and "Open on <site>". A late
  `load` clears it.
- Instagram and Bluesky show their own "removed / not found" page, and a
  refused request still fires `load` (browser error page), so neither needs
  or can use a guess. A missing height message is not treated as failure.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

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

@@ -315,6 +315,33 @@ export const EmbedIframeStatic = style([
   },
 ]);
 
+// Shown over (or instead of) an embed that hung or that the provider says is gone.
+export const EmbedNotice = style([
+  DefaultReset,
+  {
+    position: 'absolute',
+    inset: 0,
+    display: 'flex',
+    flexDirection: 'column',
+    alignItems: 'center',
+    justifyContent: 'center',
+    gap: config.space.S200,
+    padding: config.space.S300,
+    textAlign: 'center',
+    backgroundColor: color.Surface.Container,
+    color: color.Surface.OnContainer,
+  },
+]);
+
+export const EmbedNoticeStatic = style([
+  EmbedNotice,
+  {
+    position: 'static',
+    minHeight: toRem(120),
+    borderRadius: config.radii.R300,
+  },
+]);
+
 export const EmbedPlaceholder = style([
   DefaultReset,
   {