fix(dev): quiet the /api proxy when the local bridge isn't running

ui/vite.config.ts

@@ -8,12 +8,33 @@ export default defineConfig({
   plugins: [react()],
   base: './',
   build: { outDir: 'dist' },
-  // In `npm run dev`, proxy the API to the local bridge so the UI works against the
-  // real backend. In production the bundle is served by the bridge itself (same origin),
-  // so `/api/*` is already relative.
+  // In `npm run dev` (and `tauri dev`), proxy the API to the local bridge so the UI works against
+  // the real backend when it is running. The bridge is OPTIONAL in dev: without it the app falls
+  // back to demo/mock, so a connection refused is expected, not an error. Quiet the noisy
+  // ECONNREFUSED stack to a single line and answer 503 so the request never hangs. In production
+  // the bundle is served by the bridge itself (same origin), so `/api/*` is already relative.
   server: {
     proxy: {
-      '/api': 'http://localhost:4762',
+      '/api': {
+        target: 'http://localhost:4762',
+        changeOrigin: true,
+        configure: (proxy) => {
+          let warned = false
+          proxy.on('error', (err, _req, res) => {
+            if ((err as NodeJS.ErrnoException).code === 'ECONNREFUSED') {
+              if (!warned) {
+                console.log('[dev] local bridge (konclave serve) not running — the app uses demo/mock data')
+                warned = true
+              }
+              const r = res as { writableEnded?: boolean; headersSent?: boolean; writeHead?: (n: number) => void; end?: (s?: string) => void }
+              if (r && typeof r.writeHead === 'function' && !r.headersSent && !r.writableEnded) {
+                r.writeHead(503)
+                r.end?.('local bridge not running')
+              }
+            }
+          })
+        },
+      },
     },
   },
   // Vitest owns the unit tests under src/. The Playwright e2e specs live in e2e/ and are