Add firmware update button and update endpoint

html/index.html

@@ -272,7 +272,7 @@
             <div class="button-row">
                 <!-- <button onclick="fetch('/tare')">Tank Full</button> -->
                 <button onclick="tankFullProtected()">Tank Full</button>
-                <button onclick="fetch('/calibrate')">Calibrate</button>
+                <button onclick="fetch('/update')">Firmware</button>
             </div>
         </div>