feat: decompose frontend into native ES modules (zero bundlers, zero npm)

app/static/sw.js

@@ -1,13 +1,18 @@
 // Service Worker for Notes English PWA (Stage 2)
 // Strategy: Cache-first for static assets, Network-first for API calls.
 
-const CACHE_NAME = 'notes-v2.2';
+const CACHE_NAME = 'notes-v5.0';
 const OFFLINE_URL = '/';
 
 const PRECACHE_URLS = [
   '/',
   '/static/css/style.css',
   '/static/js/app.js',
+  '/static/js/modules/api.js',
+  '/static/js/modules/ui.js',
+  '/static/js/modules/dictionary.js',
+  '/static/js/modules/srs.js',
+  '/static/js/modules/reader.js',
   '/manifest.json',
 ];