feat: Tiptap替换为Vditor + MD存储重构 + Shortcode卡片扩展

- 编辑器：Tiptap 替换为 Vditor 3.11.2（wysiwyg 模式，本地托管 5.8MB，去 CDN）
- 存储：Body 字段从 HTML 改为 Markdown，去除 BodyHTML，新增 Excerpt 列表摘要
- 安全：去除 bluemonday 依赖（MD 纯文本无 XSS 风险），go.mod 已清理
- Shortcode：新增 [zone:type:params] 扩展语法，支持活动/游戏/投票/资源卡片
- 图片上传：POST /api/posts/upload-image 直接返回 Vditor 原生响应格式
- 草稿：localStorage 键名改为 draft_post_body_md，与旧 HTML 草稿隔离
- 详情页：Vditor.method.min.js 客户端 MD → HTML 渲染，去 highlight.js CDN
- 样式：去 Tiptap 样式 ~190 行，精简工具栏 CSS，新增卡片样式
- 文档：vditor-migration-plan.md 完整记录迁移决策、架构、用法

templates/MetaLab-2026/static/vditor/dist/images/img-loading.svg

@@ -0,0 +1 @@
+<svg width="160px"  height="160px"  xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" preserveAspectRatio="xMidYMid" class="lds-double-ring" style="background: none;"><circle cx="50" cy="50" ng-attr-r="{{config.radius}}" ng-attr-stroke-width="{{config.width}}" ng-attr-stroke="{{config.c1}}" ng-attr-stroke-dasharray="{{config.dasharray}}" fill="none" stroke-linecap="round" r="20" stroke-width="4" stroke="#3b3e43" stroke-dasharray="31.41592653589793 31.41592653589793" transform="rotate(111.924 50 50)"><animateTransform attributeName="transform" type="rotate" calcMode="linear" values="0 50 50;360 50 50" keyTimes="0;1" dur="1.5s" begin="0s" repeatCount="indefinite"></animateTransform></circle><circle cx="50" cy="50" ng-attr-r="{{config.radius2}}" ng-attr-stroke-width="{{config.width}}" ng-attr-stroke="{{config.c2}}" ng-attr-stroke-dasharray="{{config.dasharray2}}" ng-attr-stroke-dashoffset="{{config.dashoffset2}}" fill="none" stroke-linecap="round" r="15" stroke-width="4" stroke="#d23f31" stroke-dasharray="23.561944901923447 23.561944901923447" stroke-dashoffset="23.561944901923447" transform="rotate(-111.924 50 50)"><animateTransform attributeName="transform" type="rotate" calcMode="linear" values="0 50 50;-360 50 50" keyTimes="0;1" dur="1.5s" begin="0s" repeatCount="indefinite"></animateTransform></circle></svg>