Add adaptive FPS and honest device reachability during streaming

DDP uses fire-and-forget UDP, so when a WiFi device becomes overwhelmed
by sustained traffic, sends appear successful while the device is
actually unreachable. This adds:

- HTTP liveness probe (GET /json/info, 2s timeout) every 10s during
  streaming, exposed as device_streaming_reachable in target state
- Adaptive FPS (opt-in): exponential backoff when device is unreachable,
  gradual recovery when it stabilizes — finds sustainable send rate
- Honest health checks: removed the lie that forced device_online=true
  during streaming; now runs actual health checks regardless
- Target editor toggle, FPS display shows effective rate when throttled,
  health dot reflects streaming reachability, red highlight when
  unreachable
- Auto-backup scheduling support in settings modal

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/templates/modals/target-editor.html

@@ -84,6 +84,18 @@
                     <input type="range" id="target-editor-keepalive-interval" min="0.5" max="5.0" step="0.5" value="1.0" oninput="document.getElementById('target-editor-keepalive-interval-value').textContent = this.value">
                 </div>
 
+                <div class="form-group" id="target-editor-adaptive-fps-group">
+                    <div class="label-row">
+                        <label for="target-editor-adaptive-fps" data-i18n="targets.adaptive_fps">Adaptive FPS:</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="targets.adaptive_fps.hint">Automatically reduce send rate when the device becomes unresponsive, and gradually recover when it stabilizes. Recommended for WiFi devices with weak signal.</small>
+                    <label class="settings-toggle">
+                        <input type="checkbox" id="target-editor-adaptive-fps">
+                        <span class="settings-toggle-slider"></span>
+                    </label>
+                </div>
+
                 <div id="target-editor-error" class="error-message" style="display: none;"></div>
             </form>
         </div>