fix(desktop): use RENDERER_VITE_API_URL for renderer API host

Read API host from import.meta.env.RENDERER_VITE_API_URL instead of
hardcoding it, so staging/dev builds point to the correct backend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/desktop/src/renderer/src/service/request.ts

@@ -1,7 +1,7 @@
 import { useAuthStore } from '../stores/auth'
 
-// Backend API host — change this when switching environments
-const API_HOST = 'https://api.multica.ai'
+// Backend API host — driven by RENDERER_VITE_API_URL env var (set in .env.production / .env.development)
+const API_HOST = import.meta.env.RENDERER_VITE_API_URL || 'https://api.multica.ai'
 
 /**
  * Fetch request wrapper for desktop app.

apps/desktop/src/renderer/src/vite-env.d.ts

@@ -1 +1,5 @@
 /// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly RENDERER_VITE_API_URL?: string
+}