feat(desktop): preserve error.code via DesktopError class in unwrap (P3-52)

Callers can now catch DesktopError and inspect err.code to distinguish
auth/validation/unknown failures; plain Error fallback kept for message.

src/renderer/lib/desktop.ts

@@ -1,8 +1,15 @@
 import type { DesktopResult } from "@shared/contracts";
 
+export class DesktopError extends Error {
+  constructor(public readonly code: string, message: string) {
+    super(message);
+    this.name = "DesktopError";
+  }
+}
+
 export function unwrap<T>(result: DesktopResult<T>): T {
   if (!result.ok) {
-    throw new Error(result.error.message);
+    throw new DesktopError(result.error.code, result.error.message);
   }
   return result.data;
 }