Add software LiPo UV protection with energy-save boot.

Detect under-voltage on GPIO1/11, persist UV state in NVS, enter a minimal
power mode without ESP-NOW, and recover after charge; expose LED mode 6
battery-low for host testing via goTool.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/webui/index.html

@@ -471,7 +471,7 @@
           <div class="card-body">
             <p class="text-muted small mb-3">
               Modi: <code>clear</code>, <code>color</code> (ganzer Ring), <code>progress</code> (0–100&nbsp;%),
-              <code>digit</code> (0–10), <code>blink</code>, <code>find-me</code>.
+              <code>digit</code> (0–10), <code>blink</code>, <code>find-me</code>, <code>battery-low</code>.
               Ziel: Master (<code>client_id=0</code>), ein Slave oder alle Slaves (Broadcast).
             </p>
             <div class="row g-3 align-items-end">
@@ -484,6 +484,7 @@
                   <option value="digit">Ziffer/Symbol</option>
                   <option value="blink">Blink</option>
                   <option value="find-me">Find me</option>
+                  <option value="battery-low">Battery low</option>
                 </select>
               </div>
               <div class="col-md-4">