Add service worker with Workbox: cache manifest, image cache, font cache, skip_waiting

- sw.js with network-first strategy, versioned cache, auto-purge old caches
- Workbox: cache_manifest, image_cache (30 days, 200 entries), font_cache (30 days, 10 entries)
- skip_waiting + clients.claim for instant updates
- CSP nonce for SW registration script
- Remove sw.js and workbox from .gitignore

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -39,9 +39,7 @@ node_modules/
 ###> spomky-labs/pwa-bundle ###
 /public/idb
 /public/pwa
-/public/workbox
 /public/favicon.ico
 /public/site.webmanifest
 /public/site.*.webmanifest
-/public/sw.js
 ###< spomky-labs/pwa-bundle ###