[web] Fix running dev server with remote owntone server; update docs

web-src/vite.config.js

@@ -1,6 +1,15 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
 
+// Support for setting the OwnTone server URL as env var VITE_OWNTONE_URL
+// in development mode.
+// E. g. start the DEV server with
+//
+//     VITE_OWNTONE_URL=https://owntone.local:3689 npm run serve
+//
+// will connect the web interface with a remote OwnTone server.
+const owntoneUrl = process.env.VITE_OWNTONE_URL ?? 'http://localhost:3689'
+
 // https://vitejs.dev/config/
 export default defineConfig({
   resolve: { alias: { '@': '/src' } },
@@ -18,13 +27,13 @@ export default defineConfig({
   server: {
     proxy: {
       '/api': {
-        target: 'http://localhost:3689'
+        target: owntoneUrl
       },
       '/artwork': {
-        target: 'http://localhost:3689'
+        target: owntoneUrl
       },
       '/stream.mp3': {
-        target: 'http://localhost:3689'
+        target: owntoneUrl
       }
     }
   }