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/image-viewer/ImageViewer.tsx

@@ -7,6 +7,7 @@ import * as css from './ImageViewer.css';
 import { useZoom } from '../../hooks/useZoom';
 import { usePan } from '../../hooks/usePan';
 import { downloadMedia } from '../../utils/matrix';
+import { MobileTouchTarget } from '../../styles/mobile.css';
 
 export type ImageViewerProps = {
   alt: string;
@@ -35,7 +36,13 @@ export const ImageViewer = as<'div', ImageViewerProps>(
       >
         <Header className={css.ImageViewerHeader} size="400">
           <Box grow="Yes" alignItems="Center" gap="200">
-            <IconButton size="300" radii="300" onClick={requestClose} aria-label="Close">
+            <IconButton
+              size="300"
+              radii="300"
+              className={MobileTouchTarget}
+              onClick={requestClose}
+              aria-label="Close"
+            >
               <Icon size="50" src={Icons.ArrowLeft} />
             </IconButton>
             <Text size="T300" truncate>
@@ -48,12 +55,18 @@ export const ImageViewer = as<'div', ImageViewerProps>(
               outlined={zoom < 1}
               size="300"
               radii="Pill"
+              className={MobileTouchTarget}
               onClick={zoomOut}
               aria-label="Zoom Out"
             >
               <Icon size="50" src={Icons.Minus} />
             </IconButton>
-            <Chip variant="SurfaceVariant" radii="Pill" onClick={() => setZoom(zoom === 1 ? 2 : 1)}>
+            <Chip
+              variant="SurfaceVariant"
+              radii="Pill"
+              className={MobileTouchTarget}
+              onClick={() => setZoom(zoom === 1 ? 2 : 1)}
+            >
               <Text size="B300">{Math.round(zoom * 100)}%</Text>
             </Chip>
             <IconButton
@@ -61,6 +74,7 @@ export const ImageViewer = as<'div', ImageViewerProps>(
               outlined={zoom > 1}
               size="300"
               radii="Pill"
+              className={MobileTouchTarget}
               onClick={zoomIn}
               aria-label="Zoom In"
             >
@@ -70,6 +84,7 @@ export const ImageViewer = as<'div', ImageViewerProps>(
               variant="Primary"
               onClick={handleDownload}
               radii="300"
+              className={MobileTouchTarget}
               before={<Icon size="50" src={Icons.Download} />}
             >
               <Text size="B300">{t('Organisms.ImageViewer.download')}</Text>

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

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,
   {

src/app/components/url-preview/UrlPreviewCard.tsx

@@ -15,6 +15,7 @@ import {
   config,
 } from 'folds';
 import { ImageOverlay } from '../ImageOverlay';
+import { MobileTouchTarget } from '../../styles/mobile.css';
 import { AsyncStatus, useAsyncCallback } from '../../hooks/useAsyncCallback';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { UrlPreview, UrlPreviewContent, UrlPreviewDescription, UrlPreviewImg } from './UrlPreview';
@@ -726,6 +727,7 @@ function TwitterCard({
             size="300"
             radii="300"
             variant="SurfaceVariant"
+            className={MobileTouchTarget}
             onClick={() => setExpanded(false)}
             aria-label="Collapse post"
           >
@@ -780,6 +782,7 @@ function TwitterCard({
             <Chip
               variant="Secondary"
               radii="Pill"
+              className={MobileTouchTarget}
               onClick={() => setExpanded(true)}
               before={<Icon size="50" src={Icons.Play} />}
             >
@@ -1249,6 +1252,7 @@ function MediaEmbedCard({
                 <Chip
                   variant="Secondary"
                   radii="Pill"
+                  className={MobileTouchTarget}
                   onClick={enterFullscreen}
                   aria-label="Fullscreen"
                 >
@@ -1259,6 +1263,7 @@ function MediaEmbedCard({
                 size="300"
                 radii="300"
                 variant="SurfaceVariant"
+                className={MobileTouchTarget}
                 onClick={() => setPlaying(false)}
                 aria-label="Close player"
               >
@@ -1407,6 +1412,7 @@ function TikTokEmbedCard({ url, prev }: { url: string; prev: IPreviewUrlResponse
               <Chip
                 variant="Secondary"
                 radii="Pill"
+                className={MobileTouchTarget}
                 onClick={enterFullscreen}
                 aria-label="Fullscreen"
               >
@@ -1416,6 +1422,7 @@ function TikTokEmbedCard({ url, prev }: { url: string; prev: IPreviewUrlResponse
                 size="300"
                 radii="300"
                 variant="SurfaceVariant"
+                className={MobileTouchTarget}
                 onClick={() => setPlaying(false)}
                 aria-label="Close player"
               >
@@ -2069,7 +2076,14 @@ export const UrlPreviewCard = as<'div', { url: string; ts: number }>(
     // player chrome / tweet content isn't cramped or clipped.
     const embed = parseMediaEmbed(url, window.location.hostname);
     const wide = !!embed || isTwitterTweet(url);
-    const cardClass = wide ? previewCss.UrlPreviewWide : undefined;
+    // Twitter/Twitch/TikTok(fallback) cards render header/thumbnail beside content
+    // in the card flex row; stack them on phones (no-op for the single-column
+    // embed cards). Desktop keeps the row layout.
+    const stackOnMobile = isTwitter(url) || isTwitch(url) || isTikTok(url);
+    const cardClass =
+      [wide && previewCss.UrlPreviewWide, stackOnMobile && previewCss.StackOnMobile]
+        .filter(Boolean)
+        .join(' ') || undefined;
 
     const renderContent = (prev: IPreviewUrlResponse): React.ReactNode => {
       // Embeddable media (YouTube/Vimeo/TikTok/Dailymotion/Streamable/Twitch/