fix: allow e2e tests to reuse existing vite server on port conflict

Set reuseExistingServer:true in playwright config so acceptance gates
don't fail when port 41700 is already occupied by a prior test run.

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

frontend/playwright.config.ts

@@ -20,7 +20,7 @@ export default defineConfig({
     command:
       "pnpm exec vite --config vite.config.ts --host 127.0.0.1 --port 41700 --strictPort",
     url: "http://127.0.0.1:41700/@vite/client",
-    reuseExistingServer: false,
+    reuseExistingServer: true,
     timeout: 120_000,
     cwd: frontendRoot,
   },