feat: add per-layer LED range and collapsible layers to composite source

Composite layers now support optional start/end LED range (toggleable)
and reverse flag, making composite a superset of mapped source. Layers
are collapsible with animated expand/collapse and consistent 0.85rem
font sizing. Delete button restyled as ghost icon button.

Also includes minor dashboard CSS overflow fixes.

server/src/wled_controller/static/css/base.css

@@ -115,6 +115,7 @@ body {
 
 html {
     background: var(--bg-color);
+    overflow-x: hidden;
     overflow-y: scroll;
     scroll-behavior: smooth;
     scrollbar-gutter: stable;