feat(demo): reset the demo instance every 20 minutes with a countdown

DEMO_RESET_MINUTES (default 20, 0 = off) resets the database to the
sample data. Demo accounts get fixed IDs so sessions survive; requests
hold a read lock so the reset never runs under a half-finished request
(streams excluded). DEMO_RESET goes to every connection, /auth/status
announces the next reset. The app shows "✨ Demo · Reset in MM:SS" in
place of the version, then clears cached answers, remounts the views,
reloads in the background and shows a short notice; if no event
arrives (long polling) it asks the server after the countdown.

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

frontend/src/App.tsx

@@ -51,7 +51,8 @@ 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 { outboxCount, clearOfflineData } from './lib/offline';
+import { DemoCountdown } from './components/DemoCountdown';
 import { UserSettingsView } from './features/settings/UserSettingsView';
 import { AdminSettingsView } from './features/settings/AdminSettingsView';
 import { HelpView } from './features/help/HelpView';
@@ -122,6 +123,12 @@ export const App: React.FC = () => {
     instanceName: 'MindSift',
   });
   const [user, setUser] = useState<User | null>(null);
+  // Demo mode: when the next reset happens (local clock), and a counter that remounts all views after it
+  const [demoDeadline, setDemoDeadline] = useState<number | null>(null);
+  const [dataEpoch, setDataEpoch] = useState(0);
+  const [demoNotice, setDemoNotice] = useState<string | null>(null);
+  const lastDemoResetRef = useRef(0);
+  const handleDemoResetRef = useRef<() => void>(() => {});
   // Link from the password reset mail (?resetToken=…); removed from the address bar right away
   const [resetToken, setResetToken] = useState<string | null>(() => {
     try {
@@ -551,11 +558,50 @@ export const App: React.FC = () => {
     }
   }, [user?.id]);
 
+  useEffect(() => {
+    setDemoDeadline(
+      authStatus.demoMode && typeof authStatus.demoResetInSeconds === 'number'
+        ? Date.now() + authStatus.demoResetInSeconds * 1000
+        : null
+    );
+  }, [authStatus.demoMode, authStatus.demoResetInSeconds]);
+
+  // The demo database was reset: drop cached answers of the old data, remount the views (they load
+  // fresh data), refresh counters and the countdown. Sessions stay valid (fixed demo account IDs).
+  const handleDemoReset = useCallback(() => {
+    if (Date.now() - lastDemoResetRef.current < 30000) return; // event and countdown may both report it
+    lastDemoResetRef.current = Date.now();
+    void clearOfflineData();
+    setDataEpoch((n) => n + 1);
+    refreshAll();
+    api.getAuthStatus().then(setAuthStatus).catch(() => {});
+    setDemoNotice('Demo-Instanz wurde zurückgesetzt');
+    window.setTimeout(() => setDemoNotice(null), 4000);
+  }, [refreshAll]);
+  handleDemoResetRef.current = handleDemoReset;
+
+  // Countdown reached zero but no DEMO_RESET arrived (e.g. long polling): ask the server shortly after
+  const handleDemoCountdownElapsed = useCallback(() => {
+    window.setTimeout(() => {
+      api
+        .getAuthStatus()
+        .then((st) => {
+          if ((st.demoResetInSeconds ?? 0) > 60) handleDemoResetRef.current();
+          else setAuthStatus(st);
+        })
+        .catch(() => {});
+    }, 5000);
+  }, []);
+
   // Refresh stats on task/doc/list real-time events (counters in the navigation)
   useEffect(() => {
     const handleWs = (e: any) => {
       const msg = e.detail;
       if (!msg) return;
+      if (msg.event === 'DEMO_RESET') {
+        handleDemoResetRef.current();
+        return;
+      }
       if (
         typeof msg.event === 'string' &&
         ['TASK_', 'DOC_', 'SPACE_', 'ITEM_', 'LIST_'].some((prefix) => msg.event.startsWith(prefix))
@@ -742,7 +788,16 @@ export const App: React.FC = () => {
                 <span className="font-bold text-base text-main tracking-tight leading-none">
                   {authStatus.instanceName || 'MindSift'}
                 </span>
-                <span className="text-[9px] text-muted font-mono leading-tight mt-0.5">{APP_VERSION}</span>
+                {demoDeadline ? (
+                  <DemoCountdown
+                    deadline={demoDeadline}
+                    minutes={authStatus.demoResetMinutes}
+                    onElapsed={handleDemoCountdownElapsed}
+                    className="text-[9px] mt-0.5"
+                  />
+                ) : (
+                  <span className="text-[9px] text-muted font-mono leading-tight mt-0.5">{APP_VERSION}</span>
+                )}
               </div>
               <ChevronDown
                 className={`w-4 h-4 text-muted transition-transform duration-150 ${
@@ -817,7 +872,16 @@ export const App: React.FC = () => {
               <span className="font-bold text-sm sm:text-base text-main tracking-tight leading-none">
                 {authStatus.instanceName || 'MindSift'}
               </span>
-              <span className="text-[10px] text-muted font-mono leading-tight mt-0.5">{APP_VERSION}</span>
+              {demoDeadline ? (
+                <DemoCountdown
+                  deadline={demoDeadline}
+                  minutes={authStatus.demoResetMinutes}
+                  onElapsed={handleDemoCountdownElapsed}
+                  className="text-[10px] mt-0.5"
+                />
+              ) : (
+                <span className="text-[10px] text-muted font-mono leading-tight mt-0.5">{APP_VERSION}</span>
+              )}
             </div>
           </div>
 
@@ -1053,11 +1117,19 @@ export const App: React.FC = () => {
 
       <MobileHeaderMenu activeTabLabel={activeTabLabelMap[activeTab]} />
       <OfflineBanner />
+      {demoNotice && (
+        <div
+          role="status"
+          className="fixed left-1/2 -translate-x-1/2 bottom-[calc(5.5rem+env(safe-area-inset-bottom))] sm:bottom-4 z-[60] px-3 py-1.5 rounded-full border border-accent/40 bg-panel-elevated text-main text-[11px] font-medium shadow-panel"
+        >
+          ✨ {demoNotice}
+        </div>
+      )}
 
       {/* 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">
         <ViewErrorBoundary onReset={() => setActiveTab('triage')}>
-          <section>
+          <section key={dataEpoch}>
             {activeTab === 'triage' && (
               <TriageDeck
                 deck={deck}