Update CLAUDE.md and fix final Tortoise handler return value

- Add dashboard development commands (just dashboard, mix phx.server)
- Document Dashboard.Application and Dashboard.MqttSubscriber components
- Add comprehensive dashboard section with MQTT configuration details
- Include critical implementation notes for Tortoise handler return values
- Fix handle_message to return {:ok, state} instead of [] to prevent crashes

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

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

dashboard/lib/dashboard/mqtt_subscriber.ex

@@ -63,7 +63,7 @@ defmodule Dashboard.MqttSubscriber do
     {:ok, state}
   end
   
-  def handle_message(topic, payload, _state) do
+  def handle_message(topic, payload, state) do
     topic_parts = if is_binary(topic), do: String.split(topic, "/"), else: topic
     case topic_parts do
       ["systant", hostname, "stats"] ->
@@ -83,7 +83,7 @@ defmodule Dashboard.MqttSubscriber do
       _ ->
         :ok
     end
-    []
+    {:ok, state}
   end
 
   @impl true