fix: guard APRS send_message when disconnected

lib/aprsme/is/is.ex

@@ -189,7 +189,10 @@ defmodule Aprsme.Is do
   end
 
   def send_message(message) do
-    GenServer.call(__MODULE__, {:send_message, message})
+    case Process.whereis(__MODULE__) do
+      nil -> {:error, :not_connected}
+      _pid -> GenServer.call(__MODULE__, {:send_message, message})
+    end
   end
 
   # Server methods

test/aprsme/is_test.exs

@@ -195,6 +195,12 @@ defmodule Aprsme.IsTest do
     end
   end
 
+  describe "send_message/1 when GenServer is not running" do
+    test "returns not_connected instead of crashing" do
+      assert Aprsme.Is.send_message("test message") == {:error, :not_connected}
+    end
+  end
+
   describe "handle_info(:aprsme_no_message_timeout, ...)" do
     test "ignores timeout when socket is nil" do
       state = build_state(%{socket: nil})