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/read-receipt-avatars/ReadReceiptAvatars.tsx

@@ -22,6 +22,7 @@ import { stopPropagation } from '../../utils/keyboard';
 import { useModalStyle } from '../../hooks/useModalStyle';
 import { useMemberAvatar } from '../../hooks/useMemberAvatar';
 import { useRoomMembersChange } from '../../hooks/useRoomMemberChange';
+import { MobileTouchTarget } from '../../styles/mobile.css';
 import * as css from './ReadReceiptAvatars.css';
 
 const MAX_DISPLAY = 5;
@@ -92,7 +93,7 @@ export function ReadReceiptAvatars({
         onClick={() => setOpen(true)}
         title={tooltipNames}
         aria-label={tooltipNames}
-        className={css.ReceiptTrigger}
+        className={`${css.ReceiptTrigger} ${MobileTouchTarget}`}
       >
         {/* Pill wrapper ensures visibility on any wallpaper/background */}
         <span