Fix nonlocal scoping in CSS processing, move brightness source emoji to dropdown items

Add nonlocal declarations for _u16_a, _u16_b, _i32 in nested functions
_blend_u16 and _apply_corrections — Python treats augmented assignments
(*=, +=, >>=) as local variable bindings, causing UnboundLocalError
that prevented any frames from being sent to devices.

Move 🔢 emoji from brightness source label to dropdown option items.

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

server/src/wled_controller/static/js/features/kc-targets.js

@@ -363,11 +363,9 @@ function _populateKCBrightnessVsDropdown(selectedId = '') {
     // Keep the first "None" option, remove the rest
     while (sel.options.length > 1) sel.remove(1);
     _cachedValueSources.forEach(vs => {
-        const typeIcons = { static: '📊', animated: '🔄', audio: '🎵' };
-        const icon = typeIcons[vs.source_type] || '🔢';
         const opt = document.createElement('option');
         opt.value = vs.id;
-        opt.textContent = `${icon} ${vs.name}`;
+        opt.textContent = `🔢 ${vs.name}`;
         sel.appendChild(opt);
     });
     sel.value = selectedId || '';