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>

.agents/skills/halo-plugin-dev/references/plugin-structure.md

@@ -0,0 +1,46 @@
+# Plugin Project Structure
+
+## Typical Directory Layout
+
+```
+my-halo-plugin/
+├── ui/                              # Frontend source (Vue + TypeScript)
+│   ├── src/
+│   │   ├── assets/
+│   │   ├── views/                   # Page components
+│   │   ├── components/              # Reusable components
+│   │   └── index.ts                 # Plugin entry (definePlugin)
+│   ├── package.json
+│   ├── tsconfig.json
+│   ├── vite.config.ts | rsbuild.config.ts
+│   └── ...
+├── src/main/
+│   ├── java/
+│   │   └── com/example/myplugin/
+│   │       └── MyPlugin.java        # Entry class extending BasePlugin
+│   └── resources/
+│       ├── plugin.yaml              # Plugin manifest (required)
+│       ├── console/                 # Built frontend output (main.js + style.css)
+│       ├── extensions/              # YAML extension declarations
+│       ├── templates/               # Thymeleaf templates (optional, for theme integration)
+│       └── static/                  # Static assets served at /plugins/{name}/assets (optional)
+├── build.gradle                     # Gradle build config
+├── gradle.properties
+├── settings.gradle
+├── gradlew
+└── README.md
+```
+
+## Backend (`src/main/java/`)
+
+- **Entry class**: One class extending `run.halo.app.plugin.BasePlugin`, annotated with `@Component`. It is the only lifecycle entry point.
+- **Spring features supported**: Core IoC, WebFlux reactive stack, Testing. Standard annotations: `@Component`, `@Service`, `@Repository`, `@Configuration`, `@Controller`, etc.
+- **Resources**: `src/main/resources/plugin.yaml` is mandatory. `src/main/resources/extensions/` holds YAML declarations for custom extensions, role templates, settings, etc.
+
+## Frontend (`ui/` or `console/`)
+
+- **Entry file**: `ui/src/index.ts` (or `console/src/index.ts`) exports a default object created by `definePlugin()`.
+- **Build output**: compiled to `src/main/resources/console/main.js` + `style.css`. Halo merges all plugin JS/CSS into global bundles.
+- **Build tools**: Vite or Rsbuild via `@halo-dev/ui-plugin-bundler-kit`.
+
+> From Halo 2.11+, UC (User Center) shares the same plugin mechanism. `resources/console` may be renamed to `resources/ui` in future but both are compatible.