feat(room-preview): preview affordance in space lobby + explore grids

Follow-up to the room-preview work so you can actually reach a preview from the
room lists, not just via a matrix.to link:

- Space lobby (RoomItem): un-joined child rooms now show an Eye 'Preview' chip
  next to Join. It routes through the existing onOpen (data-room-id -> space/room
  path), which renders the full JoinBeforeNavigate preview card because the room
  isn't joined.
- Explore grids (Server + Featured): pass join_rule (and encryption, where the
  summary provides it) to RoomCard, so the directory/featured cards — which already
  are full preview cards — now show the join-rule chip and a Request-to-join button
  for knock rooms instead of a Join that fails.

738 tests pass, build clean.

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

src/app/pages/client/explore/Featured.tsx

@@ -70,6 +70,8 @@ export function FeaturedRooms() {
                                 name={roomSummary?.name}
                                 topic={roomSummary?.topic}
                                 memberCount={roomSummary?.num_joined_members}
+                                joinRule={roomSummary?.join_rule}
+                                encrypted={!!roomSummary?.['im.nheko.summary.encryption']}
                                 onView={navigateSpace}
                                 renderTopicViewer={(name, topic, requestClose) => (
                                   <RoomTopicViewer
@@ -101,6 +103,8 @@ export function FeaturedRooms() {
                                 name={roomSummary?.name}
                                 topic={roomSummary?.topic}
                                 memberCount={roomSummary?.num_joined_members}
+                                joinRule={roomSummary?.join_rule}
+                                encrypted={!!roomSummary?.['im.nheko.summary.encryption']}
                                 onView={navigateRoom}
                                 renderTopicViewer={(name, topic, requestClose) => (
                                   <RoomTopicViewer

src/app/pages/client/explore/Server.tsx

@@ -604,6 +604,7 @@ export function PublicRooms() {
                               topic={chunkRoom.topic}
                               memberCount={chunkRoom.num_joined_members}
                               roomType={chunkRoom.room_type}
+                              joinRule={chunkRoom.join_rule}
                               onView={
                                 chunkRoom.room_type === RoomType.Space
                                   ? navigateSpace