Fix PWA offline precaching on fresh installs

[wpgaurav]

Summary

• return HTTP 200 for the cacheable PWA offline application shell
• handle the first functionalities_pwa option save through added_option
• add regression coverage for first-save hooks and the offline response
• keep the fix in the 1.4.8 changelog

Root cause

The service worker precaches the offline URL with cache.addAll(), which rejects the previous HTTP 503 response. On a fresh installation, WordPress also fires added_option instead of updated_option, so the plugin did not register and flush the new PWA rewrites after the first settings save.

Validation

• composer check: 19 tests, 69 assertions
• distribution ZIP rebuild and integrity check
• fresh WordPress Studio install on WordPress 7.0.2 / PHP 8.2
• first PWA option creation produced working rewrites immediately
• offline shell, manifest, and service worker each returned HTTP 200