fix(ci): reuse existing server + downgrade upload-artifact to v3

Playwright's webServer config tried to spin up its own server in CI
('reuseExistingServer: !process.env.CI' = false), but the workflow
already started one manually — port 3000 was busy and Playwright
errored before any test ran. Set reuseExistingServer: true always:
Playwright reuses whatever's responsive and only runs the command
when nothing is.

Forgejo doesn't support actions/upload-artifact@v4 (GHES-not-supported
error). Downgrade to @v3.

playwright.config.js

@@ -27,7 +27,7 @@ export default defineConfig({
   webServer: {
     command: `PORT=${PORT} npm run build && PORT=${PORT} NODE_ENV=development npm run preview`,
     url: BASE_URL,
-    reuseExistingServer: !process.env.CI,
+    reuseExistingServer: true,
     env: {
       NUXT_PUBLIC_COMING_SOON: "false",
       NODE_ENV: "development",