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/state/mediaGallery.ts

@@ -0,0 +1,3 @@
+import { atom } from 'jotai';
+
+export const mediaGalleryAtom = atom<boolean>(false);