docs: add UI dev instructions and configurable API proxy target

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/vite.config.ts

@@ -1,13 +1,18 @@
 import { defineConfig } from 'vite';
 import react from '@vitejs/plugin-react';
 
+// Use VITE_API_TARGET to proxy to a remote server, e.g.:
+//   VITE_API_TARGET=https://api.cameleer.siegeln.net npm run dev
+const apiTarget = process.env.VITE_API_TARGET || 'http://localhost:8081';
+
 export default defineConfig({
   plugins: [react()],
   server: {
     proxy: {
       '/api': {
-        target: 'http://localhost:8081',
+        target: apiTarget,
         changeOrigin: true,
+        secure: false,
       },
     },
   },