feat(system): 实现注册码功能并优化用户注册流程

- 新增注册码生成和验证功能- 实现系统设置管理，包括注册功能开关
- 更新前端界面，增加系统管理和注册码相关功能
- 修改数据库结构，添加系统设置和注册码表

sql/system.sql

@@ -0,0 +1,32 @@
+-- ----------------------------
+-- Table structure for system_settings
+-- ----------------------------
+DROP TABLE IF EXISTS "system_settings";
+CREATE TABLE "system_settings" (
+  "setting_key" TEXT NOT NULL,
+  "setting_value" TEXT,
+  "description" TEXT,
+  PRIMARY KEY ("setting_key")
+);
+
+-- ----------------------------
+-- Records of system_settings
+-- ----------------------------
+INSERT INTO "system_settings" ("setting_key", "setting_value", "description") VALUES ('registration.enabled', 'true', '是否开放用户注册功能');
+
+-- ----------------------------
+-- Table structure for registration_codes
+-- ----------------------------
+DROP TABLE IF EXISTS "registration_codes";
+CREATE TABLE "registration_codes" (
+  "id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
+  "code" TEXT NOT NULL,
+  "expiry_time" TEXT NOT NULL,
+  "created_by" TEXT,
+  "created_at" TEXT DEFAULT (datetime('now','localtime'))
+);
+
+CREATE UNIQUE INDEX "uk_code"
+ON "registration_codes" (
+  "code" ASC
+);