Use host IP for e2e test API in CI

The act runner executes steps inside a container where localhost does
not reach the Docker host. Use E2E_API_URL env var (set to the host IP
192.168.1.10:8100 in CI) so both the global setup and Vite proxy can
reach the test API container.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -15,7 +15,7 @@ export default defineConfig({
   server: {
     proxy: {
       '/api': {
-        target: 'http://localhost:8000',
+        target: process.env.E2E_API_URL || 'http://localhost:8000',
         changeOrigin: true,
       },
     },