fix(editor): move bg color from textarea to overlay so highlighted text is visible

The textarea sits above the highlight overlay in paint order (same stacking
context, later in DOM). With bg-slate-950 on the textarea, its opaque
background painted over the overlay's light text — every code field (Python
Transform's script, any format:"code" or expression field) rendered as an
apparently-empty black box.

Swap: bg-slate-950 moves onto the overlay; the textarea becomes bg-transparent
and keeps its border, caret, and selection styling.

CLAUDE.md

@@ -1,3 +1,7 @@
+<!-- release-notes-config
+declined: true
+-->
+
 # VolcanicCore — project context for Claude sessions
 
 VolcanicCore is a self-hosted N8N-style workflow engine (C# backend + React frontend). Operator draws a directed graph of nodes (triggers → actions → terminals) in a browser canvas; each workflow can be activated independently. When active, trigger nodes subscribe/listen and each event fires one execution through the graph.

frontend/src/expr/ExpressionField.tsx

@@ -83,12 +83,13 @@ export const ExpressionField = forwardRef<ExpressionFieldHandle, Props>(function
         <div
           ref={overlayRef}
           aria-hidden
-          // border-transparent matches the textarea's 1px border so the
-          // padding starts at the same offset — otherwise highlights render
-          // 1px off from the caret column.
+          // The overlay carries the bg (not the textarea): the textarea sits on top
+          // in stacking order, and if it had an opaque bg it would paint over the
+          // highlighted text. border-transparent matches the textarea's 1px border
+          // so the padding starts at the same offset.
           className={
             "absolute inset-0 pointer-events-none whitespace-pre-wrap break-words font-mono text-sm " +
-            "px-3 py-2 leading-5 text-slate-100 overflow-hidden border border-transparent rounded"
+            "px-3 py-2 leading-5 text-slate-100 overflow-hidden border border-transparent rounded bg-slate-950"
           }
         >
           {tokens.map((t, i) =>
@@ -117,7 +118,7 @@ export const ExpressionField = forwardRef<ExpressionFieldHandle, Props>(function
           placeholder={placeholder}
           spellCheck={false}
           className={
-            "relative w-full rounded bg-slate-950 border border-slate-800 " +
+            "relative w-full rounded bg-transparent border border-slate-800 " +
             "px-3 py-2 outline-none focus:border-brand-500 " +
             "font-mono text-sm leading-5 resize-none " +
             "text-transparent caret-slate-100 selection:text-slate-100 selection:bg-brand-600/40"