fix: composite layer opacity/brightness widgets + CSS layout

- Fix opacity widget empty space (CSS selector .composite-layer-opacity → .composite-layer-opacity-container)
- Replace brightness select dropdown with BindableScalarWidget (slider + VS toggle)
- Legacy brightness_source_id auto-converted to BindableFloat on load
- Add .composite-layer-brightness-container CSS rule

server/src/wled_controller/api/routes/system.py

@@ -268,7 +268,8 @@ def get_system_performance(_: AuthRequired):
 
     # App-level metrics
     proc_mem = _process.memory_info()
-    app_cpu = _process.cpu_percent(interval=None)
+    # Process.cpu_percent() is per-core (0–N*100%); normalize to 0–100% scale
+    app_cpu = _process.cpu_percent(interval=None) / (psutil.cpu_count(logical=True) or 1)
     app_ram_mb = round(proc_mem.rss / 1024 / 1024, 1)
 
     gpu = None