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/handler/auth.go

@@ -21,7 +21,7 @@ func AuthVerify() nf.HandlerFunc {
 	}
 
 	return func(c *nf.Ctx) error {
-		if opt.Cfg.Auth == "" {
+		if opt.Cfg.Username == "" || opt.Cfg.Password == "" {
 			return c.Next()
 		}