feat(frontend): add text editor flow and create-file UX

- replace textarea editor with CodeMirror and syntax highlighting\n- integrate editor mode header (save button, hide unused controls)\n- fix breadcrumb/editor navigation behavior and transitions\n- add Create File action with ghost-rename flow and auto-open in editor\n- refine create-file icon shape and plus cutout alignment

frontend/package.json

@@ -18,8 +18,11 @@
     "node": ">=18.0.0"
   },
   "dependencies": {
+    "@codemirror/language-data": "^6.5.2",
+    "@codemirror/theme-one-dark": "^6.1.3",
     "@imengyu/vue3-context-menu": "^1.5.3",
     "@vueuse/core": "^14.1.0",
+    "codemirror": "^6.0.2",
     "esbuild": "^0.27.2",
     "lodash": "^4.17.23",
     "lodash-es": "^4.17.23",