feat(web)!: sign in before joining a space and drop the stranger option

Opening a space link while signed out shows the sign-in page first and keeps
the link in memory. The channel settings no longer offer the stranger switch,
and the client supports API contract major 8.

packages/core/src/session.ts

@@ -13,7 +13,6 @@ export type AuthRole = 'read' | 'write' | 'owner';
 /** 频道设置,字段名与 API 一致(snake_case)。 */
 export interface ChannelSettings {
   join_policy?: 'open' | 'invite';
-  allow_guests?: boolean;
   name?: string;
   max_members?: number;
   members_can_upload?: boolean;