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/src/components/HeaderMain.vue

@@ -2,6 +2,11 @@
   <nav class="headermain buttons">
     <template v-if="!props.editorMode">
       <UploadButton :path="props.path" />
+      <SvgButton
+        name="create-file"
+        tooltip="New file"
+        @click="() => { store.fileExplorer!.newFile() }"
+      />
       <SvgButton
         name="create-folder"
         tooltip="New folder"