feat: implement single binary build and env-based auth

- Unify login page styling with share page
- Add AGENTS.md with build commands and code style guidelines
- Add dev.sh and make.sh for development and production builds
- Implement single binary build with embedded frontend using embed.FS
- Change auth configuration from CLI flag to env variables (USHARE_USERNAME, USHARE_PASSWORD)
- Set default credentials: admin / ushare@123
- Fix static file serving for SPA routes

internal/controller/user.go

@@ -21,11 +21,11 @@ func (um *userManager) Login(username string, password string) (*model.User, err
 		now = time.Now()
 	)
 
-	if username != "admin" {
+	if username != opt.Cfg.Username {
 		return nil, errors.New("账号或密码错误")
 	}
 
-	if !tool.ComparePassword(password, opt.Cfg.Auth) {
+	if !tool.ComparePassword(password, opt.Cfg.Password) {
 		return nil, errors.New("账号或密码错误")
 	}