feat(files): add CodeMirror 6 editor with save, dirty tracking, and 15 language modes

v2/src/lib/adapters/files-bridge.ts

@@ -20,3 +20,7 @@ export function listDirectoryChildren(path: string): Promise<DirEntry[]> {
 export function readFileContent(path: string): Promise<FileContent> {
   return invoke<FileContent>('read_file_content', { path });
 }
+
+export function writeFileContent(path: string, content: string): Promise<void> {
+  return invoke<void>('write_file_content', { path, content });
+}