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,
+  };
+}