fix: return JSON errors for unmatched local API routes

Test development and built-preview route registration, preserve development-only credential editing, and document local preview configuration.

server/standalone/api-not-found.js

@@ -0,0 +1,17 @@
+/** Finish unmatched API requests before Vite's HTML fallback. Install last. */
+export function apiNotFoundPlugin() {
+  const install = (server) => {
+    server.middlewares.use('/api', (_req, res) => {
+      res.writeHead(404, {
+        'Content-Type': 'application/json',
+        'Cache-Control': 'no-store',
+      });
+      res.end(JSON.stringify({ error: 'Unknown API route' }));
+    });
+  };
+  return {
+    name: 'api-not-found',
+    configureServer: install,
+    configurePreviewServer: install,
+  };
+}

server/standalone/vite.config.js

@@ -2,6 +2,7 @@ import { fileURLToPath } from 'node:url';
 import { defineConfig, loadEnv } from 'vite';
 import { createBrowserViteConfig } from '../../build/vite.js';
 import { localProviderPlugins } from '../providers/local.js';
+import { apiNotFoundPlugin } from './api-not-found.js';
 
 const root = fileURLToPath(new URL('../../', import.meta.url));
 
@@ -12,7 +13,7 @@ export default defineConfig(({ mode }) => {
     if (process.env[key] === undefined) process.env[key] = value;
   }
   return createBrowserViteConfig({
-    plugins: localProviderPlugins(),
+    plugins: [...localProviderPlugins(), apiNotFoundPlugin()],
     googleApiKey: process.env.GOOGLE_MAPS_API_KEY,
     cesiumToken: process.env.CESIUM_ION_TOKEN,
     host: process.env.HOST,