feat: 初始化 Halo 暗色模式插件

- Halo Plugin 后端（Java/Gradle），含 DarkModePlugin 主类和测试
- Vue 3 + TypeScript 前端 UI，包含主题切换组件和设置页面
- 暗色模式 CSS 变量和覆盖样式（布局/编辑器/表单/滚动条等）
- 设计文档和调查文档
- Halo 插件/主题开发 Agent Skills

Co-Authored-By: Claude <noreply@anthropic.com>

src/main/resources/plugin.yaml

@@ -0,0 +1,22 @@
+# Refer https://docs.halo.run/developer-guide/plugin/basics/manifest
+
+apiVersion: plugin.halo.run/v1alpha1
+kind: Plugin
+metadata:
+  # The name defines how the plugin is invoked, A unique name
+  name: dark-mode
+spec:
+  enabled: true
+  requires: ">=2.25.0"
+  author:
+    name: LHY
+    website: https://github.com/LHY
+  logo: logo.png
+  homepage: https://github.com/LHY/dark-mode#readme
+  repo: https://github.com/LHY/dark-mode
+  issues: https://github.com/LHY/dark-mode/issues
+  displayName: "深色模式"
+  description: "为 Halo 后台管理面板提供深色/浅色模式切换，支持跟随系统、手动切换和偏好记忆"
+  license:
+    - name: "GPL-3.0"
+      url: "https://github.com/LHY/dark-mode/blob/main/LICENSE"