feat: 官网 + 管理端 + Gin/GORM 后端首个完整版本

- web/: 产品官网（Hero 事件流终端、功能矩阵、架构、快速开始、下载、博客 + Markdown 详情页），青瓷绿双主题
- admin/: 内容管理（JWT 登录、文章分页/搜索/CRUD、草稿与发布），embed 挂 /admin
- server/: Gin + GORM，MySQL（DSN 走 .env，SQLite 兜底）；规范落地：sundynix_ 表前缀、字符串雪花主键、snake_case 列名、统一响应信封、公共分页参数
- 双 SPA embed 单二进制部署，make build 一键出包

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

admin/src/components/theme-provider.tsx

@@ -0,0 +1,63 @@
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useState,
+  type ReactNode,
+} from 'react'
+
+type Theme = 'light' | 'dark' | 'system'
+
+interface ThemeContextValue {
+  theme: Theme
+  resolved: 'light' | 'dark'
+  setTheme: (t: Theme) => void
+}
+
+const ThemeContext = createContext<ThemeContextValue | null>(null)
+
+const STORAGE_KEY = 'sundynix-theme'
+
+function systemTheme(): 'light' | 'dark' {
+  return window.matchMedia('(prefers-color-scheme: dark)').matches
+    ? 'dark'
+    : 'light'
+}
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>(
+    () => (localStorage.getItem(STORAGE_KEY) as Theme) ?? 'system',
+  )
+  const [resolved, setResolved] = useState<'light' | 'dark'>(() =>
+    theme === 'system' ? systemTheme() : theme,
+  )
+
+  useEffect(() => {
+    const apply = () => {
+      const r = theme === 'system' ? systemTheme() : theme
+      setResolved(r)
+      document.documentElement.classList.toggle('dark', r === 'dark')
+    }
+    apply()
+    const mq = window.matchMedia('(prefers-color-scheme: dark)')
+    mq.addEventListener('change', apply)
+    return () => mq.removeEventListener('change', apply)
+  }, [theme])
+
+  const setTheme = (t: Theme) => {
+    localStorage.setItem(STORAGE_KEY, t)
+    setThemeState(t)
+  }
+
+  return (
+    <ThemeContext.Provider value={{ theme, resolved, setTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  )
+}
+
+export function useTheme() {
+  const ctx = useContext(ThemeContext)
+  if (!ctx) throw new Error('useTheme must be used within ThemeProvider')
+  return ctx
+}