ui: 验证码改为 4 位，登录页图片移到输入框左侧

captcha 服务 CAPTCHA_LEN 调整为 4 并适配字符间距；登录页验证码图片与输入框换序，同一行展示更紧凑。

static/public/login.html

@@ -18,8 +18,8 @@
 					<input type="password" id="loginPassword" placeholder="请输入密码">
 					<span id="pwdTip"></span>
 					<label>验证码：</label>
-					<input type="text" id="captchaAnswer" placeholder="请输入验证码" maxlength="5" autocomplete="off" style="width:110px">
-					<img id="captchaImg" alt="加载中…" title="点击刷新验证码" style="width:120px;height:38px;vertical-align:middle;cursor:pointer;margin-left:8px;border:1px solid #ddd;border-radius:4px;box-sizing:border-box">
+					<img id="captchaImg" alt="加载中…" title="点击刷新验证码" style="width:120px;height:38px;vertical-align:middle;cursor:pointer;margin-right:8px;border:1px solid #ddd;border-radius:4px;box-sizing:border-box">
+					<input type="text" id="captchaAnswer" placeholder="请输入验证码" maxlength="4" autocomplete="off" style="width:110px">
 					<span id="captchaTip"></span>
 				</div>
 				<div class="form-actions">