Add value sources for dynamic brightness control on LED targets

Introduces a new Value Source entity that produces a scalar float (0.0-1.0)
for dynamic brightness modulation. Three subtypes: Static (constant),
Animated (sine/triangle/square/sawtooth waveform), and Audio-reactive
(RMS/peak/beat from mono audio source). Value sources can be optionally
attached to LED targets to control brightness each frame.

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

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

@@ -33,6 +33,17 @@
                     <select id="target-editor-css-source"></select>
                 </div>
 
+                <div class="form-group">
+                    <div class="label-row">
+                        <label for="target-editor-brightness-vs" data-i18n="targets.brightness_vs">Brightness Source:</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="targets.brightness_vs.hint">Optional value source that dynamically controls brightness each frame (overrides device brightness)</small>
+                    <select id="target-editor-brightness-vs">
+                        <option value="" data-i18n="targets.brightness_vs.none">None (device brightness)</option>
+                    </select>
+                </div>
+
                 <div class="form-group" id="target-editor-fps-group">
                     <div class="label-row">
                         <label for="target-editor-fps">