refactor: replace dual-layer textarea with Monaco Editor for script body

The transparent-textarea-over-highlighted-overlay approach had
persistent cursor alignment issues. Replace with Monaco Editor
(already used in tree editor Code Mode) using built-in PowerShell
language support and our existing dark theme. Gives proper syntax
highlighting, cursor positioning, line numbers, and editing for free.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/script-editor/ScriptBodyEditor.tsx

@@ -1,5 +1,7 @@
-import { useRef, useCallback } from 'react'
-import { PowerShellHighlighter } from '@/components/scripts/PowerShellHighlighter'
+import { useCallback } from 'react'
+import Editor, { type BeforeMount } from '@monaco-editor/react'
+import { resolutionFlowTheme, THEME_ID } from '@/components/tree-editor/code-mode/resolutionFlowTheme'
+import { Spinner } from '@/components/common/Spinner'
 
 interface Props {
   value: string
@@ -8,50 +10,43 @@ interface Props {
 }
 
 export function ScriptBodyEditor({ value, onChange, disabled }: Props) {
-  const textareaRef = useRef<HTMLTextAreaElement>(null)
-  const overlayRef = useRef<HTMLDivElement>(null)
-
-  const handleScroll = useCallback(() => {
-    if (textareaRef.current && overlayRef.current) {
-      overlayRef.current.scrollTop = textareaRef.current.scrollTop
-      overlayRef.current.scrollLeft = textareaRef.current.scrollLeft
-    }
+  const handleBeforeMount: BeforeMount = useCallback((monaco) => {
+    // Register our dark theme if not already defined
+    monaco.editor.defineTheme(THEME_ID, resolutionFlowTheme)
   }, [])
 
-  const handleTab = useCallback((e: React.KeyboardEvent<HTMLTextAreaElement>) => {
-    if (e.key === 'Tab') {
-      e.preventDefault()
-      const ta = e.currentTarget
-      const start = ta.selectionStart
-      const end = ta.selectionEnd
-      const newValue = value.substring(0, start) + '    ' + value.substring(end)
-      onChange(newValue)
-      // Restore cursor position after React re-render
-      requestAnimationFrame(() => {
-        ta.selectionStart = ta.selectionEnd = start + 4
-      })
-    }
-  }, [value, onChange])
-
   return (
-    <div className="relative rounded-xl border border-border overflow-hidden">
-      {/* Highlighted overlay (read-only visual layer) — scroll synced to textarea */}
-      <div ref={overlayRef} className="absolute inset-0 pointer-events-none overflow-hidden p-4">
-        <PowerShellHighlighter script={value || ' '} className="font-label text-sm leading-[21px] whitespace-pre m-0 p-0 border-0" />
-      </div>
-
-      {/* Editable textarea (transparent text, visible caret) */}
-      <textarea
-        ref={textareaRef}
+    <div className="rounded-xl border border-border overflow-hidden">
+      <Editor
+        height="300px"
+        language="powershell"
+        theme={THEME_ID}
         value={value}
-        onChange={e => onChange(e.target.value)}
-        onScroll={handleScroll}
-        onKeyDown={handleTab}
-        disabled={disabled}
-        wrap="off"
-        spellCheck={false}
-        className="relative z-10 w-full min-h-[300px] resize-y font-label text-sm leading-[21px] bg-transparent text-transparent caret-foreground p-4 whitespace-pre overflow-auto focus:outline-none focus:ring-1 focus:ring-[rgba(6,182,212,0.2)] disabled:cursor-not-allowed disabled:opacity-50"
-        placeholder="# Enter your PowerShell script here…&#10;# Use {{ param_name }} for parameter placeholders"
+        onChange={v => onChange(v ?? '')}
+        beforeMount={handleBeforeMount}
+        loading={
+          <div className="flex h-[300px] items-center justify-center bg-card">
+            <Spinner size="sm" className="h-6 w-6 border-t-foreground" />
+          </div>
+        }
+        options={{
+          minimap: { enabled: false },
+          fontSize: 13,
+          fontFamily: "'JetBrains Mono', monospace",
+          lineNumbers: 'on',
+          wordWrap: 'on',
+          scrollBeyondLastLine: false,
+          renderLineHighlight: 'line',
+          tabSize: 4,
+          insertSpaces: true,
+          automaticLayout: true,
+          readOnly: disabled,
+          padding: { top: 12, bottom: 12 },
+          scrollbar: {
+            verticalScrollbarSize: 8,
+            horizontalScrollbarSize: 8,
+          },
+        }}
       />
     </div>
   )