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/features/lobby/RoomItem.tsx

@@ -97,6 +97,31 @@ function RoomJoinButton({ roomId, via }: RoomJoinButtonProps) {
   );
 }
 
+// Open the full preview page (JoinBeforeNavigate) for an un-joined room. onOpen
+// reads data-room-id and navigates to the space→room path, which — because the
+// room isn't joined — renders the preview card instead of the timeline.
+function RoomPreviewChip({
+  roomId,
+  onOpen,
+}: {
+  roomId: string;
+  onOpen: MouseEventHandler<HTMLButtonElement>;
+}) {
+  return (
+    <Chip
+      data-room-id={roomId}
+      onClick={onOpen}
+      variant="Secondary"
+      fill="None"
+      size="400"
+      radii="Pill"
+      aria-label="Preview room"
+    >
+      <Icon size="50" src={Icons.Eye} />
+    </Chip>
+  );
+}
+
 function RoomProfileLoading() {
   return (
     <Box grow="Yes" gap="300">
@@ -365,7 +390,10 @@ export const RoomItemCard = as<'div', RoomItemCardProps>(
                         </Chip>
                       </Box>
                     ) : (
-                      <RoomJoinButton roomId={roomId} via={content.via} />
+                      <Box shrink="No" gap="100" alignItems="Center">
+                        <RoomPreviewChip roomId={roomId} onOpen={onOpen} />
+                        <RoomJoinButton roomId={roomId} via={content.via} />
+                      </Box>
                     )
                   }
                 />
@@ -409,7 +437,12 @@ export const RoomItemCard = as<'div', RoomItemCardProps>(
                   memberCount={summary.num_joined_members}
                   suggested={content.suggested}
                   joinRule={summary.join_rule}
-                  options={<RoomJoinButton roomId={roomId} via={content.via} />}
+                  options={
+                    <Box shrink="No" gap="100" alignItems="Center">
+                      <RoomPreviewChip roomId={roomId} onOpen={onOpen} />
+                      <RoomJoinButton roomId={roomId} via={content.via} />
+                    </Box>
+                  }
                 />
               )}
             </>