fix(mobile): stack embed cards + secondary 44px touch targets (r2)

Mobile follow-ups round 2 (survey findings deferred from the mobile audit),
reviewed by 2 agents on the staged diff (both SHIP).

- URL-preview cards: the Twitch / Twitter / TikTok-fallback cards render
  their thumbnail/header BESIDE the content as direct children of the
  UrlPreview flex row, which squeezes both on a phone. Add `StackOnMobile`
  (@media max-width:750px -> flex-direction:column) scoped to those variants
  via cardClass. folds Box has no default `direction`, so the override wins
  uncontested; desktop (>750px) is unchanged. No-op for the single-column
  embed cards (MediaEmbedCard/TikTokEmbedCard).
- 44px touch targets (MobileTouchTarget, @media max-width:750px) on the
  otherwise ~28px controls: embed-player Close/Collapse/Fullscreen/View-post
  buttons; image-viewer close/zoom/download; the read-receipt "seen by" pill.

Deferred (rationale, not built): PiP resize handles + fullscreen button —
enlarging four 24px corners to 44px would swallow a ~160px mobile PiP and
block "Return to call"; presence dot is a non-interactive status indicator.

Gates: tsc 0, eslint 0, prettier clean, 856/856 tests, build ok.

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

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

@@ -23,6 +23,19 @@ export const UrlPreviewWide = style({
   width: 'min(38rem, 94vw)',
 });
 
+// The Twitch/Twitter/TikTok-fallback cards lay their header/thumbnail out BESIDE
+// the content as direct children of the UrlPreview flex row; on a phone that
+// squeezes both. Stack them vertically on narrow viewports only. `UrlPreview`'s
+// Box has no explicit direction (browser default row), so this override wins
+// with nothing to compete against, and desktop (>750px) is unchanged.
+export const StackOnMobile = style({
+  '@media': {
+    '(max-width: 750px)': {
+      flexDirection: 'column',
+    },
+  },
+});
+
 export const UrlPreviewImg = style([
   DefaultReset,
   {