refactor(desktop): AuthContext exposes userKeyState instead of device record

Replaces the per-device DeviceRecord lookup with a per-user discriminated
union (loading | needs-setup | needs-unlock | unlocked). Heartbeat block
deleted (telemetry no longer device-bound); webPush keyed by install-id.

apps/desktop/src/components/AppShell.tsx

@@ -4,6 +4,7 @@ import { Outlet } from 'react-router-dom';
 import { useAuth } from '../context/AuthContext';
 import { startConversationKeySync } from '../lib/conversationKeySync';
 import { startDeviceApprovalListener } from '../lib/deviceApproval';
+import { ensureInstallId } from '../lib/installId';
 import { ensureNotificationPermission } from '../lib/osNotify';
 import { cachedUserKey } from '../lib/userIdentity';
 import { BackupPromptBanner } from './BackupPromptBanner';
@@ -12,7 +13,7 @@ import { DeviceApprovalBanner } from './DeviceApprovalBanner';
 import { Sidebar } from './Sidebar';
 
 export function AppShell() {
-  const { session, device } = useAuth();
+  const { session } = useAuth();
   useEffect(() => {
     // Prompt once per authenticated shell mount. Module-level guard prevents
     // re-asking if the user already responded this session.
@@ -20,20 +21,25 @@ export function AppShell() {
   }, []);
 
   useEffect(() => {
-    if (!session?.user.id || !device?.id) return;
+    if (!session?.user.id) return;
     const userId = session.user.id;
-    const deviceId = device.id;
-    const stopKeySync = startConversationKeySync(userId, deviceId);
+    // Per-install id replaces the per-device record now that crypto is
+    // user-keyed. The legacy conv-key-sync and device-approval subsystems
+    // still take a "deviceId" argument for telemetry / row identity; passing
+    // the install id keeps those wires intact until they are themselves
+    // reworked in later tasks (Task 16/17/19 territory).
+    const installId = ensureInstallId();
+    const stopKeySync = startConversationKeySync(userId, installId);
     const stopApproval = startDeviceApprovalListener({
       ownUserId: userId,
-      ownDeviceId: deviceId,
+      ownDeviceId: installId,
       getPriv: () => cachedUserKey(userId),
     });
     return () => {
       stopKeySync();
       stopApproval();
     };
-  }, [session?.user.id, device?.id]);
+  }, [session?.user.id]);
 
   return (
     <div className="relative flex min-h-screen overflow-hidden bg-surface text-fg">