feat: mod platform integration (showcase, cloud slots, cloud screen, server-side unlink)

Mod:
- %config slots/pull/publish/unpublish for the four cloud slots
- %showcase [new|popular] [page] | load | copy, with number references
- %cloud opens a CloudScreen (link/unlink, slots, showcase) on vanilla widgets
- %config load IDDQD works offline (everything off except ChinaHat)
- default backend URL is now the production gateway
- shared ConfigRemoteApplier and ClientThread replace per-class copies
- %link unlink revokes the link on the server, then clears the local token

Backend:
- POST /device/revoke (RFC 7009 style self-revoke, always 204), rate limited
  to 10/min per IP at the gateway

CloudScreen is compiled but has not been opened in a running client yet.

backend/accounts-service/src/lib.rs

@@ -49,6 +49,7 @@ pub fn build_app(pool: sqlx::PgPool, cfg: &Config) -> Router {
         .route("/device/code", post(device::handlers::create_code))
         .route("/device/confirm", post(device::handlers::confirm))
         .route("/device/token", post(device::handlers::token))
+        .route("/device/revoke", post(device::handlers::revoke_current))
         .route("/device/links", get(device::handlers::list_links))
         .route(
             "/device/links/{id}",