Add dynamic brightness value source support for KC targets, fix subtab selector collision

Extend value source brightness modulation to Key Colors targets (matching LED target support).
Also fix stream subtab CSS selector collision that broke target subtab selection, and use 🔢 emoji
for value source UI elements.

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

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

@@ -35,7 +35,7 @@
 
                 <div class="form-group">
                     <div class="label-row">
-                        <label for="target-editor-brightness-vs" data-i18n="targets.brightness_vs">Brightness Source:</label>
+                        <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>