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/gateway/src/rate_limit/rules.rs

@@ -50,6 +50,7 @@ pub fn rules() -> Vec<Rule> {
         rule(Method::POST, "/device/code", Quota::per_minute(n(10)), Ip),
         // The mod polls every 2–3 s for up to 10 min: 10/min would break linking.
         rule(Method::POST, "/device/token", Quota::per_minute(n(30)), Ip),
+        rule(Method::POST, "/device/revoke", Quota::per_minute(n(10)), Ip),
         rule(Method::PUT, "/configs/*", Quota::per_minute(n(20)), Account),
         rule(
             Method::GET,