refactor: 标准化项目结构 frontend/ + backend/ + SQLite + run.sh

- 目录重构: app/ -> frontend/，与 backend/ 并列
- 数据库: 默认 SQLite（零配置），修复 DSN() bug
- run.sh: 一键启动前后端，支持 start/stop/restart/status
- Makefile: install/dev/build/lint/clean 快捷命令
- vite.config.ts: 配置 dev server 端口 5173 + API 代理到 8080
- config.yaml: SQLite 默认配置，开箱即用
- docs/: 完整开发文档（项目结构/API/数据库/规范/部署）
- 修复: GORM foreign key 关联声明
- 修复: go get gorm.io/driver/sqlite 依赖

frontend/src/lib/utils.ts

@@ -0,0 +1,28 @@
+import { clsx, type ClassValue } from "clsx"
+import { twMerge } from "tailwind-merge"
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}
+
+export function formatDistanceToNow(timestamp: number): string {
+  const now = Date.now();
+  const diff = now - timestamp;
+  
+  const seconds = Math.floor(diff / 1000);
+  const minutes = Math.floor(seconds / 60);
+  const hours = Math.floor(minutes / 60);
+  const days = Math.floor(hours / 24);
+  
+  if (seconds < 60) {
+    return '刚刚';
+  } else if (minutes < 60) {
+    return `${minutes}分钟前`;
+  } else if (hours < 24) {
+    return `${hours}小时前`;
+  } else if (days < 30) {
+    return `${days}天前`;
+  } else {
+    return new Date(timestamp).toLocaleDateString();
+  }
+}