Add CodeMirror-based script editor, verified against real hardware

New Script section: loads the device's current Lua script (GET
/api/script), edits it in a real code editor (CodeMirror 6 with Lua
syntax highlighting via @codemirror/legacy-modes -- no dedicated
@codemirror/lang-lua package exists, verified against the npm
registry), and saves it back (POST /api/script) as raw text, not
JSON, matching the device's endpoint. Not Blockly yet -- that's its
own, much larger task; this is the raw-text step toward it.

Verified end-to-end against a real ESP32-C6: load, edit, save
hot-reloads on-device within one tick, and the change survives a
power cycle.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/editor.ts

@@ -0,0 +1,28 @@
+// Thin wrapper around CodeMirror 6 for editing a device's Lua script.
+// There's no dedicated @codemirror/lang-lua package (verified against
+// the npm registry and @codemirror/legacy-modes' source) -- Lua
+// highlighting comes from the legacy StreamLanguage-based mode instead.
+
+import { EditorView, basicSetup } from 'codemirror';
+import { StreamLanguage } from '@codemirror/language';
+import { lua } from '@codemirror/legacy-modes/mode/lua';
+
+const luaLanguage = StreamLanguage.define(lua);
+
+export function createScriptEditor(parent: HTMLElement, initialDoc: string): EditorView {
+  return new EditorView({
+    parent,
+    doc: initialDoc,
+    extensions: [basicSetup, luaLanguage],
+  });
+}
+
+export function getEditorText(view: EditorView): string {
+  return view.state.doc.toString();
+}
+
+export function setEditorText(view: EditorView, text: string): void {
+  view.dispatch({
+    changes: { from: 0, to: view.state.doc.length, insert: text },
+  });
+}