feat: explore page space banners, icon-derived gradients, and space descriptions

- Redesign explore cards with banner images, overlapping icons, and frosted fade
- Extract dominant colors from space icons for dynamic banner gradients
- Add space description/banner fields to schema with migration
- Move origin label from banner overlay to content metadata row
- Support space descriptions in settings overview panel

packages/server/src/ws/handler.ts

@@ -675,6 +675,7 @@ function buildReadyPayload(userId: string): {
         id: spaceRow.id,
         name: spaceRow.name,
         icon: spaceRow.icon,
+        banner: spaceRow.banner ?? null,
         ownerId: spaceRow.ownerId,
         inviteCode: spaceRow.inviteCode,
         visibility: (spaceRow.visibility ?? 'private') as SpaceWithChannelsAndMembers['visibility'],