Enable pre-warm cache + faster Playwright config

Restore page pre-warming at HTTP server startup (was skipped) and
increase render workers from 2→4. Tighten Playwright timeouts and
run 3 workers in parallel for faster test runs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/playwright/playwright.config.js

@@ -4,12 +4,13 @@ const { defineConfig } = require('playwright/test');
 module.exports = defineConfig({
   testDir: __dirname,
   testMatch: '*.spec.js',
-  timeout: 60000,
+  timeout: 30000,
   retries: 0,
-  workers: 1,
+  workers: 3,
   use: {
     baseURL: process.env.SX_TEST_URL || 'http://localhost:8013',
     headless: true,
+    actionTimeout: 10000,
   },
   projects: [
     { name: 'chromium', use: { browserName: 'chromium' } },