fix(media-gallery): dock as a flex sibling like MembersDrawer (was floating)

The real reason the gallery didn't look or function like the Members drawer
or Saved Messages: it was a position:fixed overlay floating over the timeline,
mounted from RoomViewHeader. Now it docks into the room layout row exactly like
MembersDrawer.

- new mediaGalleryAtom (mirrors bookmarksPanelAtom) holds the open state
- RoomViewHeader toggles the atom instead of local useState and no longer
  renders the panel
- Room.tsx renders <MediaGallery> as a flex sibling of the timeline with a
  vertical Line separator on desktop and key={room.roomId} to reset per room
- MediaGallery.css: static width on desktop, position:fixed inset:0 full-screen
  only on mobile (identical strategy to MembersDrawer.css); root Box shrink="No"

The panel now shares the row with the timeline instead of overlapping it.

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

src/app/features/room/Room.tsx

@@ -2,9 +2,11 @@ import React, { useCallback } from 'react';
 import { Box, Line } from 'folds';
 import { useParams } from 'react-router-dom';
 import { isKeyHotkey } from 'is-hotkey';
-import { useAtomValue } from 'jotai';
+import { useAtomValue, useSetAtom } from 'jotai';
 import { RoomView } from './RoomView';
 import { MembersDrawer } from './MembersDrawer';
+import { MediaGallery } from './MediaGallery';
+import { mediaGalleryAtom } from '../../state/mediaGallery';
 import { ScreenSize, useScreenSizeContext } from '../../hooks/useScreenSize';
 import { useSetting } from '../../state/hooks/settings';
 import { settingsAtom } from '../../state/settings';
@@ -31,6 +33,8 @@ export function Room() {
   const callEmbed = useCallEmbed();
 
   const [isDrawer] = useSetting(settingsAtom, 'isPeopleDrawer');
+  const galleryOpen = useAtomValue(mediaGalleryAtom);
+  const setGalleryOpen = useSetAtom(mediaGalleryAtom);
   const [hideActivity] = useSetting(settingsAtom, 'hideActivity');
   const screenSize = useScreenSizeContext();
   const powerLevels = usePowerLevels(room);
@@ -78,6 +82,14 @@ export function Room() {
             <CallChatView />
           </>
         )}
+        {!callView && galleryOpen && (
+          <>
+            {screenSize === ScreenSize.Desktop && (
+              <Line variant="Background" direction="Vertical" size="300" />
+            )}
+            <MediaGallery key={room.roomId} room={room} onClose={() => setGalleryOpen(false)} />
+          </>
+        )}
         {!callView && isDrawer && (
           <>
             {screenSize === ScreenSize.Desktop && (