Add log file handling and viewing capabilities

This commit introduces the ability to handle log files through the API and
provides a new log viewing page. The API now supports GET and DELETE methods
for log file operations, allowing retrieval and deletion of log contents.
A new log.html page has been added for viewing logs in the browser, with
automatic refresh every 5 seconds and styling based on log levels.

The app.go file has been updated to include a GetLogFilepath function that
retrieves or generates the log file path. The NewLogger function now accepts
a file parameter to enable file logging. The main.js file has been updated
to include a link to the new log.html page.

This enhancement improves the observability and management of the application
by providing real-time access to logs and the ability to clear them directly
from the web interface.

www/main.js

@@ -47,6 +47,7 @@ nav li {
         <li><a href="index.html">Streams</a></li>
         <li><a href="add.html">Add</a></li>
         <li><a href="editor.html">Config</a></li>
+        <li><a href="log.html">Log</a></li>
     </ul>
 </nav>
 ` + document.body.innerHTML;