ci: fix runner image (node:20), add Playwright smoke tests

- Change runner to node:20 (has bash) instead of node:20-alpine
- Add tests/smoke.spec.js: 7 tests covering title, welcome screen,
  COMARCA_PATHS count (43), Lluçanès/Moianès presence, and
  filter counts (coastal=12, interior=21, mountain=10)
- Add playwright.config.js
- Fix CI workflow: use 'serve' for static server, proper SSH key setup

playwright.config.js

@@ -0,0 +1,16 @@
+// playwright.config.js
+const { defineConfig, devices } = require('@playwright/test');
+
+module.exports = defineConfig({
+  testDir: './tests',
+  timeout: 30_000,
+  retries: 0,
+  reporter: 'list',
+  use: {
+    baseURL: process.env.APP_URL || 'http://localhost:9090',
+    headless: true,
+  },
+  projects: [
+    { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
+  ],
+});