refactor(stage-5): modularize style.css into domain stylesheets and update service worker precache

app/static/sw.js

@@ -1,12 +1,19 @@
 // Service Worker for Notes English PWA (Stage 2)
 // Strategy: Cache-first for static assets, Network-first for API calls.
 
-const CACHE_NAME = 'notes-v6.6';
+const CACHE_NAME = 'notes-v6.7';
 const OFFLINE_URL = '/';
 
 const PRECACHE_URLS = [
   '/',
   '/static/css/style.css',
+  '/static/css/base.css',
+  '/static/css/components.css',
+  '/static/css/dictionary.css',
+  '/static/css/srs.css',
+  '/static/css/reader.css',
+  '/static/css/grammar_cefr.css',
+  '/static/css/lesson.css',
   '/static/js/app.js',
   '/static/js/modules/api.js',
   '/static/js/modules/ui.js',