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>

ui/build.gradle

@@ -0,0 +1,33 @@
+plugins {
+    id 'base'
+    id "com.github.node-gradle.node" version "7.1.0"
+}
+
+group 'run.halo.darkmode.ui'
+
+tasks.register('pnpmBuild', PnpmTask) {
+    group = 'build'
+    description = 'Build the UI project using pnpm'
+    args = ['build']
+    dependsOn tasks.named('pnpmInstall')
+    inputs.dir(layout.projectDirectory.dir('src'))
+    inputs.files(fileTree(
+        dir: layout.projectDirectory,
+        includes: ['*.cjs', '*.ts', '*.js', '*.json', '*.yaml']))
+    outputs.dir(layout.buildDirectory.dir('dist'))
+}
+
+tasks.register('pnpmCheck', PnpmTask) {
+    group = 'verification'
+    description = 'Run unit tests for the UI project using pnpm'
+    args = ['test:unit']
+    dependsOn tasks.named('pnpmInstall')
+}
+
+tasks.named('check') {
+    dependsOn tasks.named('pnpmCheck')
+}
+
+tasks.named('assemble') {
+    dependsOn tasks.named('pnpmBuild')
+}