fix: 商业核算页未登录时显示登录提示，不暴露内容

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/Projects.vue

@@ -1,5 +1,12 @@
 <template>
   <div class="projects-page">
+    <!-- Login prompt -->
+    <div v-if="!auth.isLoggedIn" class="login-prompt">
+      <div class="commercial-icon">💼</div>
+      <p>登录后可使用商业核算功能</p>
+      <button class="btn-primary" @click="ui.openLogin()">登录 / 注册</button>
+    </div>
+    <template v-else>
     <!-- Header -->
     <div class="commercial-header">
       <div class="commercial-icon">💼</div>
@@ -228,6 +235,7 @@
         </div>
       </div>
     </div>
+    </template>
   </div>
 </template>
 
@@ -951,6 +959,14 @@ function formatDate(d) {
   color: #6b6375;
 }
 
+.login-prompt {
+  text-align: center;
+  padding: 60px 20px;
+  color: #6b6375;
+}
+.login-prompt .commercial-icon { font-size: 48px; margin-bottom: 12px; }
+.login-prompt p { margin-bottom: 16px; font-size: 15px; }
+
 .empty-hint {
   text-align: center;
   color: #b0aab5;