feat(auth): add device session tracking with configurable limits

Track authenticated sessions per device (parsing user-agent for desktop
vs mobile via mileusna/useragent), enforce configurable concurrent device
limits (Auth.DeviceLimits), and surface device status and "remove other
devices" management in the account dialog. Adds device/session context to
the auth module stores and exposes limits through the API.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/domain/auth.ts

@@ -63,3 +63,8 @@ export type AccountDevice = {
   client: string;
   lastSeenAt: string;
 };
+
+export type AccountDeviceLimits = {
+  desktop: number;
+  mobile: number;
+};