Remote: reboot / shutdown the Pi from the Output tab

Add POST /api/power/:action (reboot|shutdown) that runs `sudo systemctl
reboot|poweroff` (sudo is passwordless on the appliance), replying before the
host tears down. Two red buttons in the Output tab use a two-tap confirm to
avoid accidental power-offs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/index.html

@@ -126,6 +126,10 @@
       </select>
       <button id="rec" class="wide">🔴 Avvia registrazione</button>
       <div id="rec-status" class="hint"></div>
+      <label class="lbl">Dispositivo</label>
+      <button id="pw-reboot" class="wide danger">🔄 Riavvia Raspberry</button>
+      <button id="pw-shutdown" class="wide danger">⏻ Spegni Raspberry</button>
+      <div id="pw-status" class="hint">Tocca due volte per confermare. Dopo lo spegnimento, per riaccendere stacca e riattacca l'alimentazione.</div>
     </section>
   </main>