Rename word library nav to memory curve and show it by default

Open /words on the memory visualization view first and align dashboard, bottom nav, and practice back navigation labels with the new naming.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/layouts/MainLayout.vue

@@ -9,8 +9,8 @@
         首页
       </router-link>
       <router-link to="/words" class="nav-item">
-        <span class="nav-icon">📚</span>
-        词库
+        <span class="nav-icon">📈</span>
+        记忆曲线
       </router-link>
       <router-link to="/coach" class="nav-item">
         <span class="nav-icon">💬</span>

frontend/src/pages/Dashboard.vue

@@ -58,7 +58,7 @@ onMounted(async () => {
       </div>
 
       <div class="quick-actions">
-        <router-link to="/words" class="btn btn-outline">单词库</router-link>
+        <router-link to="/words" class="btn btn-outline">记忆曲线</router-link>
         <router-link to="/quiz" class="btn btn-primary">开始每日训练</router-link>
         <router-link to="/spell" class="btn btn-outline">拼写练习</router-link>
         <router-link to="/coach" class="btn btn-outline">记忆对话</router-link>

frontend/src/pages/GraphPractice.vue

@@ -193,7 +193,7 @@ watch(practiceMode, () => {
 <template>
   <div class="practice-page">
     <header class="practice-header">
-      <button type="button" class="btn-back" @click="finishAndBack">← 返回词库</button>
+      <button type="button" class="btn-back" @click="finishAndBack">← 返回记忆曲线</button>
       <h1 class="practice-title">关联练习</h1>
       <span class="practice-progress">{{ progressText }}</span>
     </header>

frontend/src/pages/WordLibrary.vue

@@ -48,8 +48,8 @@ function refreshGraphPracticeResults() {
 }
 
 const viewTabs = [
-  { key: 'list', label: '单词列表' },
   { key: 'memory', label: '记忆曲线' },
+  { key: 'list', label: '单词列表' },
 ]
 
 const tabs = [
@@ -62,7 +62,7 @@ const tabs = [
 
 const PAGE_SIZE = 20
 
-const activeView = ref('list')
+const activeView = ref('memory')
 const activeTab = ref('')
 const words = ref<Word[]>([])
 const currentPage = ref(1)
@@ -276,13 +276,18 @@ watch(
 )
 
 onMounted(() => {
-  loadWords()
+  if (activeView.value === 'memory') {
+    refreshGraphPracticeResults()
+    loadViz()
+  } else {
+    loadWords()
+  }
 })
 </script>
 
 <template>
   <div class="page">
-    <h1 class="page-title">单词库</h1>
+    <h1 class="page-title">记忆曲线</h1>
 
     <div class="view-tabs">
       <button