Restore bottom nav entry for memory coach dialogue.

Keep the training FAB while adding a visible 对话 tab so users can
reach /coach without opening the floating menu.

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

frontend/src/layouts/MainLayout.vue

@@ -15,6 +15,10 @@
         <span class="nav-icon">📚</span>
         词库
       </router-link>
+      <router-link to="/coach" class="nav-item">
+        <span class="nav-icon">💬</span>
+        对话
+      </router-link>
       <router-link to="/settings" class="nav-item">
         <span class="nav-icon">⚙️</span>
         设置

frontend/src/styles.css

@@ -154,10 +154,12 @@ a {
   display: flex;
   flex-direction: column;
   align-items: center;
-  font-size: 11px;
+  font-size: 10px;
   color: var(--muted);
-  padding: 4px 12px;
+  padding: 4px 6px;
   text-decoration: none;
+  min-width: 0;
+  flex: 1;
 }
 
 .nav-item.router-link-active {