feat(web): 新增更新日志、服务条款、隐私政策页面；关于页更新日志链接修正

- web/dist/changelog/ 版本历史页，收录 v1.0.0–v1.0.29 全部发版记录
- web/dist/terms/ 服务条款页（付费/授权/数据/免责等）
- web/dist/privacy/ 隐私政策页（数据收集/使用/共享/用户权利等）
- 全站页脚补充 更新日志、服务条款、隐私政策三个链接
- about_screen 「更新日志」按钮链接由 /downloads/ 改为 /changelog/
- 新增 backend/cmd/issue 工具，用于签发指定门店的授权码 token

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/cmd/issue/main.go

@@ -0,0 +1,72 @@
+// issue signs a license token for a specific shop.
+// Usage: go run ./cmd/issue -shop 1 -days 365 -type annual -key <base64-private-key>
+// Or use env var: LICENSE_ED25519_PRIVATE_KEY=<key> go run ./cmd/issue -shop 1 -days 365
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/wangjia/jiu/backend/internal/util"
+)
+
+func main() {
+	shopID := flag.Uint64("shop", 0, "shop ID (required)")
+	licenseID := flag.Uint64("license", 0, "license record ID (optional, 0 = omit)")
+	days := flag.Int("days", 365, "validity days; 0 = perpetual (no expiry)")
+	licType := flag.String("type", "annual", "license type: trial | annual | lifetime")
+	maxDevices := flag.Int("devices", 3, "max devices")
+	privKey := flag.String("key", "", "Ed25519 private key (base64); falls back to LICENSE_ED25519_PRIVATE_KEY env")
+	flag.Parse()
+
+	if *shopID == 0 {
+		fmt.Fprintln(os.Stderr, "error: -shop is required")
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	key := *privKey
+	if key == "" {
+		key = os.Getenv("LICENSE_ED25519_PRIVATE_KEY")
+	}
+	if key == "" {
+		fmt.Fprintln(os.Stderr, "error: provide -key or set LICENSE_ED25519_PRIVATE_KEY")
+		os.Exit(1)
+	}
+
+	now := time.Now()
+	payload := util.LicensePayload{
+		ShopID:     *shopID,
+		Type:       *licType,
+		IssuedAt:   now.Unix(),
+		MaxDevices: *maxDevices,
+	}
+	if *licenseID > 0 {
+		payload.LicenseID = *licenseID
+	}
+	if *days > 0 {
+		exp := now.Add(time.Duration(*days) * 24 * time.Hour).Unix()
+		payload.ExpiresAt = &exp
+	}
+
+	token, err := util.IssueLicenseToken(payload, key)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "sign failed: %v\n", err)
+		os.Exit(1)
+	}
+
+	out, _ := json.MarshalIndent(payload, "", "  ")
+	fmt.Println("=== License Token ===")
+	fmt.Println(token)
+	fmt.Println()
+	fmt.Println("=== Payload ===")
+	fmt.Println(string(out))
+	if payload.ExpiresAt != nil {
+		fmt.Printf("\nExpires: %s\n", time.Unix(*payload.ExpiresAt, 0).Format("2006-01-02 15:04:05"))
+	} else {
+		fmt.Println("\nExpires: never (perpetual)")
+	}
+}