Two review agents found the preview card degrades to 'id + logo + Join' for real
reasons. Fixes:
- **via was dropped at the summary fetch** (root cause). RoomSummaryLoader now
accepts + forwards `via` to getRoomSummary (and keys the query on it);
JoinBeforeNavigate passes viaServers; the lobby Preview chip carries data-via and
Lobby.handleOpenRoom appends it to the navigated URL. Without this, previews of
rooms the HS isn't already in came back sparse/404.
- **No loading/error state** -> RoomSummaryLoader now surfaces {loading,error};
JoinBeforeNavigate shows a Spinner while loading instead of the degraded card.
- **Room id leaked as name AND topic AND header** -> stop using the raw `!id` as
the topic fallback (show 'No description'); name falls back to canonical_alias
then alias-localpart; the page header shows the summary name.
- **Richer, membership-aware card**: forward canonical_alias (shown under the name),
world_readable ('Readable' badge), and membership -> the button now shows Accept
invite / Requested / Banned correctly instead of a Join that lies; a 'hero' layout
for the single-preview page (primary Join button, unclamped topic). Removed dead
`|| undefined ||`.
IRoomSummary re-adds canonical_alias (the SDK type Omit<>s it though MSC3266
returns it). 738 tests pass, build clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>