test: increase Playwright timeout from 15s to 20s (reduce flaky failures)

The 15s timeout was too tight for the dev server under concurrent test
load, causing widespread toBeVisible failures on first attempt across
40+ smoke tests. Increasing to 20s gives the dev server adequate time
to render without masking real regressions.

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

playwright.config.ts

@@ -2,7 +2,7 @@ import { defineConfig } from '@playwright/test'
 
 export default defineConfig({
   testDir: './tests/smoke',
-  timeout: 15_000,
+  timeout: 20_000,
   retries: 2,
   workers: 1,
   use: {