Add min brightness threshold to LED targets

New per-target property: when effective output brightness
(max pixel value × device/source brightness) falls below
the threshold, LEDs turn off completely. Useful for cutting
dim flicker in audio-reactive and ambient setups.

Threshold slider (0–254) in target editor, badge on card,
hot-swap to running processors, persisted in storage.

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

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

@@ -44,6 +44,18 @@
                     </select>
                 </div>
 
+                <div class="form-group" id="target-editor-brightness-threshold-group">
+                    <div class="label-row">
+                        <label for="target-editor-brightness-threshold">
+                            <span data-i18n="targets.min_brightness_threshold">Min Brightness Threshold:</span>
+                            <span id="target-editor-brightness-threshold-value">0</span>
+                        </label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="targets.min_brightness_threshold.hint">Effective output brightness (pixel brightness × device/source brightness) below this value turns LEDs off completely (0 = disabled)</small>
+                    <input type="range" id="target-editor-brightness-threshold" min="0" max="254" value="0" oninput="document.getElementById('target-editor-brightness-threshold-value').textContent = this.value">
+                </div>
+
                 <div class="form-group" id="target-editor-fps-group">
                     <div class="label-row">
                         <label for="target-editor-fps">