fix(downloader): nil-safe isAria2RPCDisconnected to stop a startup panic

isAria2RPCDisconnected called err.Error() unconditionally, so a nil error
panicked (nil pointer deref). findSeed reaches it with a nil error whenever
tellStatus succeeds but the status isn't a seed — which crashed the downloader at
startup while restoring a retained seed from the ledger. Guard nil -> false.

cmd/internal/engine/aria2.go

@@ -1001,6 +1001,9 @@ func (a Aria2) reconnect(ctx context.Context, aria **arigo.Client) error {
 }
 
 func isAria2RPCDisconnected(err error) bool {
+	if err == nil {
+		return false
+	}
 	return errors.Is(err, rpc2.ErrShutdown) || errors.Is(err, io.ErrClosedPipe) || strings.Contains(err.Error(), "connection is shut down")
 }
 

cmd/internal/engine/engine_test.go

@@ -230,6 +230,14 @@ func TestAria2SeedTimeMinutes(t *testing.T) {
 	}
 }
 
+func TestIsAria2RPCDisconnectedNilSafe(t *testing.T) {
+	// findSeed/findTask call this with a nil error when tellStatus succeeded but
+	// the status wasn't what we wanted; it must not panic on err.Error().
+	if isAria2RPCDisconnected(nil) {
+		t.Fatal("nil error must not be treated as disconnected")
+	}
+}
+
 func TestShouldAttachExistingAria2Task(t *testing.T) {
 	// 'interrupted' (how restart resumes a task) must attach to the session-
 	// restored download rather than re-add (which duplicates + orphans it).