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/pages/ConversationPage.tsx

@@ -37,6 +37,7 @@ import { useCall } from '../context/CallContext';
 import { useConversationsContext } from '../context/ConversationsContext';
 import { collectConversationAttachments, createPollPayload } from '../lib/conversationFeatures';
 import { compressImages } from '../lib/imageCompress';
+import { ensureInstallId } from '../lib/installId';
 import { searchCachedMessages } from '../lib/messageCache';
 import type { OutboxItem } from '../lib/messageOutbox';
 import { useConversationMessages } from '../lib/useConversationMessages';
@@ -61,7 +62,7 @@ const scrollPositions = new Map<string, { scrollTop: number; stickToBottom: bool
 export function ConversationPage() {
   const { t } = useTranslation(['app', 'errors']);
   const { id } = useParams<{ id: string }>();
-  const { session, device } = useAuth();
+  const { session } = useAuth();
   const { conversations, setActiveConversation, markRead, unread } = useConversationsContext();
 
   const conversation = useMemo(
@@ -75,7 +76,7 @@ export function ConversationPage() {
     useConversationMessages({
       conversationId: id,
       userId: session?.user.id,
-      deviceId: device?.id,
+      deviceId: ensureInstallId(),
     });
   const messageIds = useMemo(() => messages.map((m) => m.id), [messages]);
   const {