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/components/RecipeDetailOverlay.vue

@@ -1,5 +1,5 @@
 <template>
-  <div class="detail-overlay" @click.self="$emit('close')">
+  <div class="detail-overlay" @mousedown.self="$emit('close')">
     <div class="detail-panel">
       <!-- ==================== CARD VIEW ==================== -->
       <div v-if="viewMode === 'card'" class="detail-card-view">