fix: subscribe push sample on activation

[KirtiRamchandani]

Fixes #1408

Summary

• Subscribe the cookbook.push service worker during activate so the sample covers non-interactive registration.
• Make subscribeUserVisibleOnlyFalse() safe to rerun by logging an existing subscription instead of trying to create a duplicate.
• Update the README flow so DevTools is used to copy or reprint the subscription data, not to create the first subscription manually.

Tests

• npx prettier functional-samples/cookbook.push/README.md functional-samples/cookbook.push/manifest.json --check
• npx eslint functional-samples/cookbook.push/background.js
• git diff --check
• node -e "JSON.parse(require(''fs'').readFileSync(''functional-samples/cookbook.push/manifest.json'',''utf8'')); console.log(''manifest ok'')"