feat(power): add setting to show current status instead of action

Reddit report: the header power toggle switches correctly but
displays "backwards" - the button shows the action a click would
perform (e.g. "Aus" while the printer is on), not the printer's
actual current state, which some users find counterintuitive.

Both readings are legitimate, so this adds a per-printer opt-in
(Settings -> Power Switch -> "Aktuellen Status statt Aktion
anzeigen") rather than flipping the default for everyone.
power_status_inverted=0 (default) keeps today's behavior; =1 mirrors
the actual on/off state instead. The backend's power-status parsing
is untouched - only _applyHeaderPowerState() picks which state to
display, so the click handler still always toggles the real state
regardless of display mode.

web/themes/default/index.html

@@ -566,6 +566,10 @@
                 <input type="text" id="s-power-status-url" placeholder="http://192.168.x.x/cm?cmnd=Power">
                 <small id="lbl-power-hint" style="color:var(--txt2)"></small>
               </div>
+              <div class="modal-field" style="flex-direction:row;align-items:center;gap:10px">
+                <input type="checkbox" id="s-power-status-inverted" style="width:auto;margin:0">
+                <label id="lbl-power-status-inverted" style="margin:0;cursor:pointer" for="s-power-status-inverted">Aktuellen Status statt Aktion anzeigen</label>
+              </div>
             </div>
           </div>