make more aspects of API & UI hosting configurable

This is necessary to create proper dev & production builds of the app.

unwind-ui/vite.config.ts

@@ -3,5 +3,11 @@ import vue from '@vitejs/plugin-vue'
 
 // https://vitejs.dev/config/
 export default defineConfig({
-  plugins: [vue()]
+  base: process.env.BASE_URL || "/",
+  define: {
+    "process.env.API_URL": JSON.stringify(
+      process.env.API_URL || "http://localhost:8000/api/",
+    ),
+  },
+  plugins: [vue()],
 })