feat: Update PWA service worker for authentication

- Add authentication pages to static cache (/auth/login, /auth/register, /auth/error)
- Implement special handling for authentication API requests (network-only with offline error)
- Add authentication page caching strategy (cache-first with network fallback)
- Prevent caching of authentication errors (401, 403)
- Enhance offline page with online detection and auto-redirect
- Add authentication notice to offline page
- Update cache version to v2 for proper invalidation
- Create PWA_AUTH_GUIDE.md documentation

Task: 19. Update PWA service worker for authentication
Requirements: 4.4

public/sw.js

@@ -1,13 +1,16 @@
 // Service Worker for Ultimate Lists App
 // Implements caching strategies for offline functionality
 
-const CACHE_NAME = 'ultimate-lists-v1';
-const RUNTIME_CACHE = 'ultimate-lists-runtime-v1';
+const CACHE_NAME = 'ultimate-lists-v2';
+const RUNTIME_CACHE = 'ultimate-lists-runtime-v2';
 
 // Static assets to cache on install
 const STATIC_ASSETS = [
   '/',
   '/offline',
+  '/auth/login',
+  '/auth/register',
+  '/auth/error',
   '/manifest.json',
   '/icon-192.svg',
   '/icon-512.svg',
@@ -58,6 +61,12 @@ self.addEventListener('fetch', (event) => {
     return;
   }
 
+  // Handle authentication-related requests
+  if (isAuthRequest(url)) {
+    event.respondWith(handleAuthRequest(request));
+    return;
+  }
+
   // Cache-first strategy for static assets
   if (isStaticAsset(url)) {
     event.respondWith(cacheFirst(request));
@@ -90,6 +99,58 @@ function isPageRequest(url) {
   return url.origin === self.location.origin && !url.pathname.includes('.');
 }
 
+// Helper: Check if request is authentication-related
+function isAuthRequest(url) {
+  return url.pathname.startsWith('/auth/') || 
+         url.pathname.startsWith('/api/auth/');
+}
+
+// Strategy: Handle authentication requests
+async function handleAuthRequest(request) {
+  const url = new URL(request.url);
+  
+  // For authentication API calls, always try network first
+  if (url.pathname.startsWith('/api/auth/')) {
+    try {
+      const response = await fetch(request);
+      return response;
+    } catch (error) {
+      console.error('Authentication API request failed (offline):', error);
+      // Return a JSON error response for API calls
+      return new Response(
+        JSON.stringify({ 
+          error: 'Authentication requires an internet connection. Please check your connection and try again.' 
+        }),
+        {
+          status: 503,
+          statusText: 'Service Unavailable',
+          headers: { 'Content-Type': 'application/json' }
+        }
+      );
+    }
+  }
+  
+  // For authentication pages, use cache-first with network fallback
+  const cache = await caches.open(CACHE_NAME);
+  const cached = await cache.match(request);
+  
+  try {
+    const response = await fetch(request);
+    if (response.ok) {
+      cache.put(request, response.clone());
+    }
+    return response;
+  } catch (error) {
+    console.error('Authentication page request failed:', error);
+    // Return cached version if available
+    if (cached) {
+      return cached;
+    }
+    // Otherwise redirect to offline page
+    return caches.match('/offline');
+  }
+}
+
 // Strategy: Cache-first (for static assets)
 async function cacheFirst(request) {
   const cache = await caches.open(CACHE_NAME);
@@ -117,6 +178,12 @@ async function staleWhileRevalidate(request) {
   const cached = await cache.match(request);
 
   const fetchPromise = fetch(request).then((response) => {
+    // Handle authentication errors (401, 403)
+    if (response.status === 401 || response.status === 403) {
+      // Don't cache authentication errors
+      return response;
+    }
+    
     if (response.ok) {
       cache.put(request, response.clone());
     }
@@ -141,6 +208,13 @@ async function staleWhileRevalidate(request) {
 async function networkFirst(request) {
   try {
     const response = await fetch(request);
+    
+    // Handle authentication errors (401, 403)
+    if (response.status === 401 || response.status === 403) {
+      // Don't cache authentication errors
+      return response;
+    }
+    
     if (response.ok) {
       const cache = await caches.open(RUNTIME_CACHE);
       cache.put(request, response.clone());