Code editor: CodeMirror 6 replaces Monaco

Node source (Python) and the JSON dialog share one CodeMirror editor with
find and replace, autocomplete, folding, indent guides, comment toggle and a
JSON lint, themed from the CSS tokens so light and dark follow the .dark class
without a second palette. Programmatic value changes no longer fire onChange,
blur ignores focus moving into the find bar, readOnly keeps selection and copy.

The editor chunk is 470 kB (157 kB gzipped) against 2.7 MB (704 kB) for Monaco,
which also pulled in a 1.2 MB JSON mode, two workers and a font. At 4x CPU on
slow 4G a node panel is ready in 5.7 s instead of 10.8 s, the JSON dialog in
1.5 s instead of 6.7 s. Typing without a key delay keeps every character.

monaco-editor, @monaco-editor/react and the dompurify override are removed.
Specs: typed code reaches the node source, a JSON setting saves only text that
parses. Docs name Tab and Ctrl-M.

Claude-Session: https://claude.ai/code/session_01SSi5AvZ7pFmySK8zy3NPMh

frontend/scripts/capture-screenshots.mjs

@@ -330,7 +330,7 @@ async function captureFlows(page, dir) {
   await page.screenshot({ path: `${dir}/app-flows.png` })
 
   await page.locator(".react-flow__node").first().click()
-  await page.waitForSelector("[data-testid=node-panel], .monaco-editor", {
+  await page.waitForSelector("[data-testid=node-panel], .cm-editor", {
     timeout: 15000,
   })
   await page.waitForTimeout(1500)