refactor(frontend): adopt GitHub-style file tree design

统一采用GitHub风格的UI设计：文件树标题改为'Code'，调整缩进和悬停样式，移除视图切换按钮，使用GitHub配色变量。

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/views/EditorView.vue

@@ -1,9 +1,7 @@
 <script setup>
 import { ref } from 'vue'
 import { defineAsyncComponent } from 'vue'
-import { useRouter } from 'vue-router'
 
-const router = useRouter()
 const MilkdownEditor = defineAsyncComponent(() => import('../components/MilkdownEditor.vue'))
 
 const markdown = ref('')
@@ -11,14 +9,6 @@ const markdown = ref('')
 
 <template>
   <div class="editor-view">
-    <div class="editor-toolbar">
-      <button class="docs-toggle" @click="router.push('/docs')" title="切换到文档模式">
-        <svg viewBox="0 0 16 16" width="16" height="16" fill="currentColor">
-          <path d="M0 2.5A1.5 1.5 0 011.5 1h2.793a.5.5 0 01.353.146l1.5 1.5a.5.5 0 00.354.146H13.5A1.5 1.5 0 0115 4.5v7.5a1.5 1.5 0 01-1.5 1.5h-11A1.5 1.5 0 011 12v-9.5z"/>
-        </svg>
-        <span class="toggle-label">文档</span>
-      </button>
-    </div>
     <MilkdownEditor v-model:markdown="markdown" />
   </div>
 </template>
@@ -31,36 +21,4 @@ const markdown = ref('')
   display: flex;
   flex-direction: column;
 }
-
-.editor-toolbar {
-  position: absolute;
-  top: 8px;
-  right: 16px;
-  z-index: 10;
-}
-
-.docs-toggle {
-  display: flex;
-  align-items: center;
-  gap: 6px;
-  padding: 6px 12px;
-  border: 1px solid var(--panel-border);
-  background: var(--app-bg);
-  color: var(--muted-text);
-  cursor: pointer;
-  border-radius: 6px;
-  font-size: 13px;
-  transition: all 0.2s ease;
-  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
-}
-
-.docs-toggle:hover {
-  color: var(--focus-ring);
-  border-color: var(--focus-ring);
-  background: var(--ghost-code-bg);
-}
-
-.toggle-label {
-  font-weight: 500;
-}
 </style>