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/lib/installId.ts

@@ -0,0 +1,19 @@
+// Per-install browser identifier. Replaces the legacy per-device crypto
+// identity for non-crypto bookkeeping (push tokens, message sender_device_id
+// metadata) where we just need a stable, opaque id for this browser/install.
+//
+// The user-key model has no device-bound crypto; the `devices` table and
+// `messages.sender_device_id` column still exist but they're now plain
+// telemetry. Using a localStorage UUID keeps the column populated without
+// any of the old key-management overhead.
+
+const KEY = 'chatapp.installId';
+
+export function ensureInstallId(): string {
+  let id = window.localStorage.getItem(KEY);
+  if (!id) {
+    id = crypto.randomUUID();
+    window.localStorage.setItem(KEY, id);
+  }
+  return id;
+}