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/db/schema.ts

@@ -19,6 +19,7 @@ export const spaces = sqliteTable('spaces', {
   id: text('id').primaryKey(),
   name: text('name').notNull(),
   icon: text('icon'),
+  banner: text('banner'),
   ownerId: text('owner_id').notNull().references(() => users.id),
   inviteCode: text('invite_code').unique(),
   visibility: text('visibility').default('private'),