Add peer-to-peer file drop

Files move directly between peers over the overlay, with no server in the
path. The receiver listens on the WireGuard address only, so the port is
unreachable from outside the tunnel, and every offer is matched to a known
peer before anything is read.

Consent is the default: an offer carries metadata alone, and no payload
moves until the receiver accepts. Policy is per profile and device-local —
off, ask, or auto-accept, with per-sender exceptions on top.

Policy and history live in the profile's preferences, so removing a profile
takes its file drop state with it. Transfers interrupted by a restart are
settled on load; nothing survives to finish them, and left alone they would
sit in the log as permanently pending.

The Android bindings pull payload bytes through a chunk-returning stream:
gomobile copies a []byte argument into a fresh Java array and never copies
it back, so a fill-my-buffer method would hand back the right length with
no data.

client/ui/tray_notify.go

@@ -41,6 +41,27 @@ func safeSendNotification(send sendFn, what string, opts notifications.Notificat
 	return nil
 }
 
+// registerNotificationResponses installs the single Wails notification-response
+// callback and fans it out per category; a second OnNotificationResponse call
+// would silently replace the first, so every category must branch here.
+func (t *Tray) registerNotificationResponses() {
+	if t.svc.Notifier == nil {
+		return
+	}
+	t.svc.Notifier.OnNotificationResponse(func(result notifications.NotificationResult) {
+		if result.Error != nil {
+			log.Debugf("notification response error: %v", result.Error)
+			return
+		}
+		switch result.Response.CategoryID {
+		case notifyCategorySessionWarning:
+			t.handleSessionWarningResponse(result.Response)
+		case notifyCategoryFileDropOffer:
+			t.handleFileDropResponse(result.Response)
+		}
+	})
+}
+
 // notifyIfDaemonOutdated probes the daemon once and fires an OS toast when it
 // is reachable but too old for this UI. A probe error means the daemon isn't
 // reachable (not outdated), so it is left to the normal connection flow.