Fix config file loading and MQTT connection error handling

Config file loading fixes:
- Fix tilde expansion in config paths (~/.config/systant/systant.toml)
- Add detailed debug logging for config file search paths
- Properly expand home directory paths with System.user_home()

MQTT connection improvements:
- Add clearer connection status logging with broker host:port
- Improve error handling in connection callback
- Better error messages when connection fails or is lost
- More detailed initial connection logging

These fixes address production deployment issues where config files
aren't found and MQTT connection errors aren't properly reported.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/lib/systant/config.ex

@@ -171,9 +171,26 @@ defmodule Systant.Config do
   end
 
   defp find_config_file do
-    @default_config_paths
-    |> Enum.map(&Path.expand/1)
-    |> Enum.find(&File.exists?/1)
+    expanded_paths = @default_config_paths |> Enum.map(&expand_config_path/1)
+    
+    Logger.debug("Searching for config files at: #{inspect(expanded_paths)}")
+    
+    case Enum.find(expanded_paths, &File.exists?/1) do
+      nil ->
+        Logger.debug("No config file found, checked: #{inspect(expanded_paths)}")
+        nil
+      path ->
+        Logger.debug("Found config file at: #{path}")
+        path
+    end
+  end
+
+  defp expand_config_path("~" <> rest) do
+    home = System.user_home()
+    Path.join(home, rest)
+  end
+  defp expand_config_path(path) do
+    Path.expand(path)
   end
 
   defp apply_env_overrides(config) do

server/lib/systant/mqtt_client.ex

@@ -16,6 +16,7 @@ defmodule Systant.MqttClient do
     mqtt_config = Systant.Config.mqtt_config(app_config)
     
     Logger.info("Starting MQTT client with config: #{inspect(mqtt_config)}")
+    Logger.info("Attempting to connect to MQTT broker at #{mqtt_config.host}:#{mqtt_config.port}")
     
     # Store both configs for later use
     state_config = %{

server/lib/systant/mqtt_handler.ex

@@ -18,11 +18,15 @@ defmodule Systant.MqttHandler do
   def connection(status, state) do
     case status do
       :up ->
-        Logger.info("MQTT connection established")
+        Logger.info("MQTT connection established successfully")
       :down ->
-        Logger.warning("MQTT connection lost")
+        Logger.error("MQTT connection lost - check MQTT broker availability and configuration")
       :terminating ->
         Logger.info("MQTT connection terminating")
+      {:error, reason} ->
+        Logger.error("MQTT connection failed: #{inspect(reason)}")
+      other ->
+        Logger.error("MQTT connection status unknown: #{inspect(other)}")
     end
     {:ok, state}
   end