feat(desktop): dedupe bind windows per platform and cap concurrency at 2

Track active bindPublishAccount calls in a per-platform map so a repeat
invocation reuses and focuses the existing window instead of spawning
a second one, and reject new platforms once 2 bind windows are already
open. Surface desktop_account_bind_limit_reached as a warning modal in
the renderer so users know to finish or close an existing authorization
before starting another.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop-client/src/renderer/lib/client-errors.ts

@@ -53,6 +53,14 @@ function presentClientError(kind: ClientActionKind, error: unknown): ClientError
     };
   }
 
+  if (message === "desktop_account_bind_limit_reached") {
+    return {
+      tone: "warning",
+      title: "授权窗口已达上限",
+      content: "当前最多同时打开 2 个授权窗口。请先完成或关闭已有授权窗口，再继续新的绑定。",
+    };
+  }
+
   if (message === "desktop_account_upsert_failed") {
     return {
       tone: "error",