feat(v1.9.0): Bootstrap 5 迁移第 2/4 轮（登录/初始化页 + 基础组件）

本轮全部为前端改动，版本号保持 1.9.0：
- CSS 加载顺序调整为 app.css → bootstrap，让 Bootstrap 覆盖表格/按钮/表单/卡片等基础组件，日历/热屏等自定义组件类不受影响
- 主题切换同时写 data-theme 与 data-bs-theme，Bootstrap 组件跟随亮/暗
- 登录页、初始化页重写为 Bootstrap card + form-control + btn-primary，垂直居中
- 客户列表搜索框加 form-control

验证：
- 无头 Chrome 实拍登录、客户列表、套餐、设置四页（shots/bs2-*.png）
- 数据库与业务逻辑零改动

后续：第 3 轮日历抽屉/追加面板/客户月历，第 4 轮大屏内部与收尾

internal/web/templates/pages/setup.html

@@ -1,25 +1,33 @@
 {{define "content"}}
-<div class="auth">
-  <div class="auth-card">
-    <div class="auth-brand"><span class="brand-mark">代</span><span>代打台</span></div>
-    <h1>创建第一个管理员</h1>
-    <p class="muted">数据库里还没有账户。设置好账号后即可开始录入套餐与客户。</p>
-    <form method="post" action="/setup" class="form">
-      <label class="field"><span>用户名</span>
-        <input type="text" name="username" required minlength="3" maxlength="32" autocomplete="username" placeholder="建议用拼音或英文名">
-      </label>
-      <label class="field"><span>显示名称</span>
-        <input type="text" name="display_name" maxlength="32" placeholder="例如：老王（可留空）">
-      </label>
-      <label class="field"><span>密码（至少 6 位）</span>
-        <input type="password" name="password" required minlength="6" autocomplete="new-password">
-      </label>
-      <label class="field"><span>再输一次密码</span>
-        <input type="password" name="password2" required minlength="6" autocomplete="new-password">
-      </label>
-      <button type="submit" class="btn btn-primary btn-block">创建并登录</button>
-    </form>
-    <p class="auth-foot muted">忘记密码只能手动改数据库，请妥善保管。</p>
+<div class="d-flex align-items-center justify-content-center" style="min-height:100vh">
+  <div class="card shadow-sm" style="width:100%;max-width:440px">
+    <div class="card-body p-4">
+      <div class="d-flex align-items-center gap-2 mb-3">
+        <span class="badge text-bg-success fs-6">代</span><span class="fw-bold">代打台</span>
+      </div>
+      <h1 class="h4 mb-1">创建第一个管理员</h1>
+      <p class="text-secondary small mb-4">数据库里还没有账户。设置好账号后即可开始录入套餐与客户。</p>
+      <form method="post" action="/setup" class="vstack gap-3">
+        <div>
+          <label class="form-label" for="username">用户名</label>
+          <input class="form-control" id="username" type="text" name="username" required minlength="3" maxlength="32" autocomplete="username" placeholder="建议用拼音或英文名">
+        </div>
+        <div>
+          <label class="form-label" for="display_name">显示名称</label>
+          <input class="form-control" id="display_name" type="text" name="display_name" maxlength="32" placeholder="例如：老王（可留空）">
+        </div>
+        <div>
+          <label class="form-label" for="password">密码（至少 6 位）</label>
+          <input class="form-control" id="password" type="password" name="password" required minlength="6" autocomplete="new-password">
+        </div>
+        <div>
+          <label class="form-label" for="password2">再输一次密码</label>
+          <input class="form-control" id="password2" type="password" name="password2" required minlength="6" autocomplete="new-password">
+        </div>
+        <button type="submit" class="btn btn-primary w-100">创建并登录</button>
+      </form>
+      <p class="text-secondary small text-center mt-3 mb-0">忘记密码只能手动改数据库，请妥善保管。</p>
+    </div>
   </div>
 </div>
 {{end}}