feat(pwa): start and keep working offline

The service worker now caches the app shell: page loads are network
first with the last index.html as fallback, hashed bundles come from the
cache, icons and manifest are refreshed in the background; API calls are
never intercepted. GET answers are kept per user in IndexedDB and served
when the server cannot be reached, so the app starts logged in with the
last known thoughts, tasks and lists. Captured thoughts and checked tasks
or list items go to an outbox that is sent in order when the connection
returns (also at start and every 30 s). A banner shows offline state and
waiting changes. Logging out clears cache and outbox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -50,6 +50,8 @@ import { TasksView } from './features/triage/TasksView';
 import { WorkspaceView } from './features/workspace/WorkspaceView';
 import { SharedListsView } from './features/lists/SharedListsView';
 import { SetupAndAuthView } from './features/auth/SetupAndAuthView';
+import { OfflineBanner } from './components/OfflineBanner';
+import { outboxCount } from './lib/offline';
 import { UserSettingsView } from './features/settings/UserSettingsView';
 import { AdminSettingsView } from './features/settings/AdminSettingsView';
 import { HelpView } from './features/help/HelpView';
@@ -525,9 +527,15 @@ export const App: React.FC = () => {
       }
     };
 
+    // Changes still waiting (server was unreachable although the device was online) are retried
+    const retry = window.setInterval(() => {
+      if (navigator.onLine && outboxCount() > 0) onOnline();
+    }, 30000);
+
     window.addEventListener('mindsift:unauthorized', onUnauthorized);
     window.addEventListener('online', onOnline);
     return () => {
+      window.clearInterval(retry);
       window.removeEventListener('mindsift:unauthorized', onUnauthorized);
       window.removeEventListener('online', onOnline);
     };
@@ -1043,6 +1051,7 @@ export const App: React.FC = () => {
       </header>
 
       <MobileHeaderMenu activeTabLabel={activeTabLabelMap[activeTab]} />
+      <OfflineBanner />
 
       {/* Main Container */}
       <main className="flex-1 max-w-6xl w-full mx-auto px-3 sm:px-4 pt-3 pb-24 sm:pb-6">