fix(frontend): real dark mode + silence dev proxy ECONNREFUSED noise

Two issues from running login.html against no Go backend:

1. Dark mode toggled the body class but didn't actually re-theme any
   AD-Vue components. The legacy panel relied on custom.min.css which
   we haven't ported. AD-Vue 4 ships its own dark algorithm — wrap
   LoginPage in <a-config-provider :theme="{ algorithm }"> driven by
   our useTheme state, and AD-Vue restyles every component for free.
   Page chrome (background, card, title) gets explicit .is-dark CSS
   since the algorithm only covers AD-Vue components.

2. Vite logged every failed proxy attempt loudly. When the Go panel
   isn't running locally that's pure noise. Added a configure()
   callback that swallows ECONNREFUSED specifically; real errors
   (timeouts, 5xx, anything else) still surface.

Both fixes are dev-experience only — production build is unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/vite.config.js

@@ -6,6 +6,27 @@ import path from 'node:path';
 // via embed.FS without reaching outside the web/ tree.
 const outDir = path.resolve(__dirname, '../web/dist');
 
+// Build a proxy config that suppresses ECONNREFUSED noise when the Go
+// backend isn't running locally. Real errors (timeouts, 5xx, etc.) still
+// surface in the Vite log.
+function makeBackendProxy(target, patterns) {
+  const config = {};
+  for (const pattern of patterns) {
+    config[pattern] = {
+      target,
+      changeOrigin: true,
+      configure(proxy) {
+        proxy.on('error', (err) => {
+          if (err.code === 'ECONNREFUSED') return;
+          // eslint-disable-next-line no-console
+          console.error('[proxy]', err);
+        });
+      },
+    };
+  }
+  return config;
+}
+
 export default defineConfig({
   plugins: [vue()],
   resolve: {
@@ -30,13 +51,12 @@ export default defineConfig({
   server: {
     port: 5173,
     strictPort: true,
-    proxy: {
-      // Proxy API calls during `npm run dev` to the local Go panel.
+    proxy: makeBackendProxy('http://localhost:2053', [
       // Patterns are anchored regex so /login.html and /index.html
       // (which Vite serves itself) are NOT forwarded — only the bare
       // backend paths and their sub-routes.
-      '^/(login|logout|getTwoFactorEnable)$': 'http://localhost:2053',
-      '^/(panel|server)(/|$)': 'http://localhost:2053',
-    },
+      '^/(login|logout|getTwoFactorEnable)$',
+      '^/(panel|server)(/|$)',
+    ]),
   },
 });