feat(邀请函): 添加二维码生成功能

- 新增二维码按钮及弹窗界面
- 实现二维码生成、预览和添加到画布功能
- 支持二维码在画布上的拖拽操作
- 添加相关样式和事件处理

css/qr.css

@@ -0,0 +1,136 @@
+/* 二维码功能样式 */
+
+/* 悬浮按钮 */
+#qr-btn {
+    position: fixed;
+    bottom: 100px;
+    left: 20px;
+    width: 44px;
+    height: 44px;
+    padding: 0;
+    border-radius: 50%;
+    background: #4a90e2;
+    color: #fff;
+    border: none;
+    font-size: 12px;
+    font-weight: bold;
+    cursor: pointer;
+    box-shadow: 0 2px 8px rgba(0,0,0,0.3);
+    z-index: 1000;
+    display: none;
+    white-space: nowrap;
+}
+
+#qr-btn.show {
+    display: block;
+}
+
+#qr-btn:hover {
+    background: #3a7bc8;
+}
+
+/* 弹窗 */
+#qr-modal {
+    display: none;
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    background: rgba(0,0,0,0.5);
+    z-index: 10000;
+    justify-content: center;
+    align-items: center;
+}
+
+#qr-modal.active {
+    display: flex;
+}
+
+.qr-box {
+    background: #fff;
+    border-radius: 8px;
+    width: 320px;
+    max-width: 90%;
+}
+
+.qr-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 15px 20px;
+    border-bottom: 1px solid #eee;
+}
+
+.qr-header h3 {
+    margin: 0;
+    font-size: 16px;
+}
+
+.qr-close {
+    background: none;
+    border: none;
+    font-size: 24px;
+    color: #999;
+    cursor: pointer;
+}
+
+.qr-body {
+    padding: 20px;
+}
+
+#qr-input {
+    width: 100%;
+    padding: 10px;
+    border: 1px solid #ddd;
+    border-radius: 4px;
+    font-size: 14px;
+    box-sizing: border-box;
+}
+
+#qr-preview {
+    margin-top: 15px;
+    min-height: 128px;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    background: #f5f5f5;
+    border-radius: 4px;
+}
+
+#qr-preview canvas {
+    display: block;
+}
+
+.qr-footer {
+    display: flex;
+    justify-content: flex-end;
+    gap: 10px;
+    padding: 15px 20px;
+    border-top: 1px solid #eee;
+}
+
+.qr-btn {
+    padding: 8px 16px;
+    border: none;
+    border-radius: 4px;
+    cursor: pointer;
+    font-size: 14px;
+}
+
+.qr-cancel {
+    background: #f0f0f0;
+    color: #333;
+}
+
+.qr-ok {
+    background: #4a90e2;
+    color: #fff;
+}
+
+/* 画布上的二维码 */
+.qr-item {
+    position: absolute;
+    cursor: move;
+    user-select: none;
+}