fix: 拖选文字时弹窗不再误关闭

所有弹窗背景关闭事件从 @click.self 改为 @mousedown.self，
避免从输入框拖选文字到弹窗外时触发关闭。

影响：LoginModal、RecipeDetailOverlay、UserMenu、TagPicker、
Projects、BugTracker、OilReference 共9处。

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

frontend/src/views/Projects.vue

@@ -207,7 +207,7 @@
     </div>
 
     <!-- Import From Recipe Modal -->
-    <div v-if="showImportModal" class="overlay" @click.self="showImportModal = false">
+    <div v-if="showImportModal" class="overlay" @mousedown.self="showImportModal = false">
       <div class="overlay-panel">
         <div class="overlay-header">
           <h3>从配方导入</h3>