Improve handling of OpenDashboardTool

* Mark the tool as optional (disabled by default)
  * Enable the tool if the dashboard is enabled, not opened and the GUI is off
  * Improve factorisation and return value/error handling

docs/02-usage/060_dashboard.md

@@ -15,11 +15,13 @@ Serena comes with built-in tools for monitoring and managing the current session
 * the **GUI tool** (disabled by default)
   
   The GUI tool is a native application window which displays logs.
+  It furthermore allows you to shut down the agent and to access the dashboard's URL (if it is running). 
 
   This is mainly supported on Windows, but it may also work on Linux; macOS is unsupported.
 
 Both can be configured in Serena's [configuration](050_configuration) file (`serena_config.yml`).
 If enabled, they will automatically be opened as soon as the Serena agent/MCP server is started.
+For the dashboard, this can be disabled if desired (see below).
 
 ## Disabling Automatic Browser Opening
 
@@ -27,5 +29,7 @@ If you prefer not to have the dashboard open automatically (e.g., to avoid focus
 by setting `web_dashboard_open_on_launch: False` in your `serena_config.yml`.
 
 When automatic opening is disabled, you can still access the dashboard by:
-* Navigating directly to `http://localhost:24282/dashboard/index.html` in your browser
-* Asking the AI to use the `open_dashboard` tool, which will open the dashboard in your default browser
+* asking the LLM to "open the Serena dashboard", which will open the dashboard in your default browser
+  (the tool `open_dashboard` is enabled for this purpose, provided that the dashboard is active, 
+  not opened by default and the GUI tool, which can provide the URL, is not enabled)
+* navigating directly to the URL (see above)