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>

server/internal/logic/auth_mapper.go

@@ -1,6 +1,8 @@
 package logic
 
 import (
+	"time"
+
 	authmodule "img_infinite_canvas/internal/modules/auth"
 	"img_infinite_canvas/internal/types"
 )
@@ -93,7 +95,7 @@ func toAPIAccountDevices(devices []authmodule.Device) []types.AccountDevice {
 			System:     device.System,
 			Browser:    device.Browser,
 			Client:     device.Client,
-			LastSeenAt: device.LastSeenAt.UTC().Format("2006-01-02 15:04(UTC)"),
+			LastSeenAt: device.LastSeenAt.UTC().Format(time.RFC3339),
 		})
 	}
 	return items