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.

.forgejo/workflows/test.yml

@@ -64,7 +64,7 @@ jobs:
         if: failure()
         run: cat /tmp/server.log || true
       - run: npx playwright test --ignore-snapshots
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v3
         if: failure()
         with:
           name: playwright-report
@@ -132,7 +132,7 @@ jobs:
         if: failure()
         run: cat /tmp/server.log || true
       - run: npx playwright test e2e/visual/
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v3
         if: failure()
         with:
           name: visual-diffs