功能: 系统更新检查（GitHub Release + Docker）

后端：
- 新增 GET /api/system/update-check，从 GitHub Releases API 获取最新版本
- 自动比较当前版本与最新版本，匹配当前平台的下载链接
- 返回版本号、更新说明、下载链接、Docker 镜像信息

前端（系统设置页重构）：
- 新增"检查更新"按钮，点击后展示更新结果
- 有新版本时显示版本号、更新说明、下载按钮、Docker 更新命令
- 新增磁盘状态卡片（总空间/已用/可用/使用率）
- 运行模式用彩色 Tag 区分（生产/开发）

server/internal/http/router.go

@@ -68,6 +68,7 @@ func NewRouter(deps RouterDependencies) *gin.Engine {
 		system := api.Group("/system")
 		system.Use(AuthMiddleware(deps.JWTManager))
 		system.GET("/info", systemHandler.Info)
+		system.GET("/update-check", systemHandler.CheckUpdate)
 
 		storageTargets := api.Group("/storage-targets")
 		storageTargets.Use(AuthMiddleware(deps.JWTManager))

server/internal/http/system_handler.go

@@ -17,3 +17,17 @@ func NewSystemHandler(systemService *service.SystemService) *SystemHandler {
 func (h *SystemHandler) Info(c *gin.Context) {
 	response.Success(c, h.systemService.GetInfo(c.Request.Context()))
 }
+
+func (h *SystemHandler) CheckUpdate(c *gin.Context) {
+	result, err := h.systemService.CheckUpdate(c.Request.Context())
+	if err != nil {
+		// 即使检查失败也返回当前版本信息
+		response.Success(c, gin.H{
+			"currentVersion": result.CurrentVersion,
+			"hasUpdate":      false,
+			"error":          err.Error(),
+		})
+		return
+	}
+	response.Success(c, result)
+}

server/internal/service/system_service.go

@@ -2,7 +2,12 @@ package service
 
 import (
 	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
 	"path/filepath"
+	"runtime"
+	"strings"
 	"syscall"
 	"time"
 
@@ -30,6 +35,82 @@ func NewSystemService(cfg config.Config, version string, startedAt time.Time) *S
 	return &SystemService{cfg: cfg, version: version, startedAt: startedAt}
 }
 
+// UpdateCheckResult 描述版本更新检查结果。
+type UpdateCheckResult struct {
+	CurrentVersion string `json:"currentVersion"`
+	LatestVersion  string `json:"latestVersion"`
+	HasUpdate      bool   `json:"hasUpdate"`
+	ReleaseURL     string `json:"releaseUrl,omitempty"`
+	ReleaseNotes   string `json:"releaseNotes,omitempty"`
+	PublishedAt    string `json:"publishedAt,omitempty"`
+	DownloadURL    string `json:"downloadUrl,omitempty"`
+	DockerImage    string `json:"dockerImage,omitempty"`
+}
+
+const githubRepoAPI = "https://api.github.com/repos/Awuqing/BackupX/releases/latest"
+
+// CheckUpdate 从 GitHub Releases 检查是否有新版本。
+func (s *SystemService) CheckUpdate(ctx context.Context) (*UpdateCheckResult, error) {
+	result := &UpdateCheckResult{
+		CurrentVersion: s.version,
+		DockerImage:    "awuqing/backupx",
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, githubRepoAPI, nil)
+	if err != nil {
+		return result, fmt.Errorf("create request: %w", err)
+	}
+	req.Header.Set("Accept", "application/vnd.github.v3+json")
+	req.Header.Set("User-Agent", "BackupX/"+s.version)
+
+	client := &http.Client{Timeout: 15 * time.Second}
+	resp, err := client.Do(req)
+	if err != nil {
+		return result, fmt.Errorf("fetch latest release: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		return result, fmt.Errorf("github api returned %d", resp.StatusCode)
+	}
+
+	var release struct {
+		TagName    string `json:"tag_name"`
+		HTMLURL    string `json:"html_url"`
+		Body       string `json:"body"`
+		Published  string `json:"published_at"`
+		Assets     []struct {
+			Name               string `json:"name"`
+			BrowserDownloadURL string `json:"browser_download_url"`
+		} `json:"assets"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&release); err != nil {
+		return result, fmt.Errorf("decode release: %w", err)
+	}
+
+	result.LatestVersion = release.TagName
+	result.ReleaseURL = release.HTMLURL
+	result.ReleaseNotes = release.Body
+	result.PublishedAt = release.Published
+
+	// 比较版本号（去 v 前缀后字符串比较）
+	current := strings.TrimPrefix(s.version, "v")
+	latest := strings.TrimPrefix(release.TagName, "v")
+	result.HasUpdate = latest > current && current != "dev"
+
+	// 匹配当前平台的下载链接
+	goos := runtime.GOOS
+	goarch := runtime.GOARCH
+	suffix := fmt.Sprintf("%s-%s.tar.gz", goos, goarch)
+	for _, asset := range release.Assets {
+		if strings.HasSuffix(asset.Name, suffix) {
+			result.DownloadURL = asset.BrowserDownloadURL
+			break
+		}
+	}
+
+	return result, nil
+}
+
 func (s *SystemService) GetInfo(_ context.Context) *SystemInfo {
 	now := time.Now().UTC()
 	info := &SystemInfo{