feat(web): 深度定制 Element Plus 主题 — 靛蓝主色/暗色模式/侧栏折叠/面包屑/登录页重绘

- 新增 styles/theme.css：CSS 变量覆盖默认主题（主色 #4F46E5、圆角、阴影、页面背景），html.dark 暗色变量
- 新增 stores/theme.ts：暗色/亮色切换，localStorage 持久化
- main.ts 引入 element-plus 暗色 css-vars 与主题文件，挂载前初始化主题
- index.html 加 SVG favicon 与防暗色闪烁内联脚本
- AdminLayout：深色可折叠侧栏、header 面包屑 + 暗色切换按钮、页面切换过渡
- UserLayout：header 打磨 + 暗色切换按钮
- LoginView/ApplyView：统一品牌渐变背景与卡片，登录页 OTP 输入行内联发送按钮、登录按钮唯一通栏主按钮（修复按钮错位）

web/src/styles/theme.css

@@ -0,0 +1,186 @@
+/* ============================================================
+ * 全局主题：通过 CSS 变量覆盖 Element Plus 默认主题
+ * 亮色为默认；html.dark 下为暗色（配合 element-plus dark css-vars）
+ * ============================================================ */
+
+:root {
+  /* 品牌主色：靛蓝（替代默认 #409EFF） */
+  --el-color-primary: #4f46e5;
+  --el-color-primary-light-3: #847eed;
+  --el-color-primary-light-5: #a7a3f2;
+  --el-color-primary-light-7: #cac8f7;
+  --el-color-primary-light-8: #dcdafa;
+  --el-color-primary-light-9: #edecfc;
+  --el-color-primary-dark-2: #3f38b7;
+
+  /* 圆角 */
+  --el-border-radius-base: 8px;
+  --el-border-radius-small: 6px;
+
+  /* 页面背景：偏冷灰白 */
+  --el-bg-color-page: #f4f5fa;
+
+  /* 阴影：更柔和 */
+  --el-box-shadow-light: 0 4px 16px rgba(15, 23, 42, 0.06);
+  --el-box-shadow: 0 10px 28px rgba(15, 23, 42, 0.1);
+
+  /* 字体栈 */
+  --el-font-family:
+    -apple-system, BlinkMacSystemFont, 'Segoe UI', 'PingFang SC',
+    'Hiragino Sans GB', 'Microsoft YaHei', 'Helvetica Neue', Arial, sans-serif;
+
+  /* 布局自定义变量 */
+  --app-sidebar-bg: #1c2333;
+  --app-sidebar-bg-active: rgba(99, 102, 241, 0.18);
+  --app-sidebar-text: rgba(255, 255, 255, 0.72);
+  --app-sidebar-text-active: #ffffff;
+  --app-sidebar-logo-bg: rgba(99, 102, 241, 0.16);
+  --app-header-height: 60px;
+}
+
+/* ---------------- 暗色模式 ---------------- */
+html.dark {
+  /* 主色在暗色下提亮一档，保证对比度 */
+  --el-color-primary: #818cf8;
+  --el-color-primary-light-3: #5a62ae;
+  --el-color-primary-light-5: #41467c;
+  --el-color-primary-light-7: #272a4a;
+  --el-color-primary-light-8: #1a1c32;
+  --el-color-primary-light-9: #14162b;
+  --el-color-primary-dark-2: #6366f1;
+
+  --el-bg-color-page: #101218;
+  --app-sidebar-bg: #161a24;
+  --app-sidebar-logo-bg: rgba(129, 140, 248, 0.16);
+}
+
+/* ---------------- 认证页（登录 / 申请）公共样式 ----------------
+ * 深色品牌背景 + 光晕，卡片浮于其上。
+ * 亮/暗模式下此背景保持一致，形成品牌页与内部页面的反差。
+ */
+.auth-page {
+  min-height: 100vh;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 24px;
+  position: relative;
+  overflow: hidden;
+  background:
+    radial-gradient(1200px 600px at 85% -10%, rgba(99, 102, 241, 0.28), transparent 60%),
+    radial-gradient(900px 500px at -10% 110%, rgba(56, 189, 248, 0.16), transparent 55%),
+    linear-gradient(135deg, #0f172a 0%, #1e293b 55%, #26224f 100%);
+}
+.auth-page::before {
+  content: '';
+  position: absolute;
+  width: 560px;
+  height: 560px;
+  border-radius: 50%;
+  background: radial-gradient(circle, rgba(99, 102, 241, 0.22), transparent 70%);
+  top: -180px;
+  right: -120px;
+  pointer-events: none;
+}
+.auth-page::after {
+  content: '';
+  position: absolute;
+  width: 480px;
+  height: 480px;
+  border-radius: 50%;
+  background: radial-gradient(circle, rgba(34, 211, 238, 0.14), transparent 70%);
+  bottom: -160px;
+  left: -100px;
+  pointer-events: none;
+}
+
+.auth-card {
+  width: 420px;
+  border-radius: 16px;
+  border: 1px solid rgba(255, 255, 255, 0.08);
+  box-shadow: 0 24px 60px rgba(2, 6, 23, 0.45);
+  position: relative;
+  z-index: 1;
+}
+html.dark .auth-card {
+  box-shadow: 0 24px 60px rgba(0, 0, 0, 0.6);
+}
+.auth-card .el-card__body {
+  padding: 28px 28px 24px;
+}
+
+/* 品牌区：图标放渐变圆角块内 */
+.auth-brand {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  flex-direction: column;
+  gap: 10px;
+  margin-bottom: 20px;
+}
+.auth-brand .brand-icon {
+  width: 52px;
+  height: 52px;
+  border-radius: 14px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: #fff;
+  background: linear-gradient(135deg, #6366f1 0%, #8b5cf6 100%);
+  box-shadow: 0 8px 20px rgba(99, 102, 241, 0.35);
+}
+.auth-brand h2 {
+  margin: 0;
+  font-size: 17px;
+  font-weight: 650;
+  letter-spacing: 0.5px;
+}
+.auth-brand .sub {
+  font-size: 12.5px;
+  color: var(--el-text-color-secondary);
+  margin-top: -6px;
+}
+
+.auth-foot {
+  margin-top: 16px;
+  text-align: center;
+  font-size: 13px;
+  color: var(--el-text-color-secondary);
+}
+
+/* 表单提交按钮：通栏主按钮 */
+.auth-submit {
+  width: 100%;
+}
+
+/* ---------------- 全局细节 ---------------- */
+/* 细滚动条 */
+::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+::-webkit-scrollbar-thumb {
+  background: rgba(120, 130, 150, 0.35);
+  border-radius: 4px;
+}
+::-webkit-scrollbar-thumb:hover {
+  background: rgba(120, 130, 150, 0.55);
+}
+::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+/* 文本选中颜色 */
+::selection {
+  background: rgba(99, 102, 241, 0.25);
+}
+
+/* 页面切换过渡（避免突兀） */
+.fade-enter-active,
+.fade-leave-active {
+  transition: opacity 0.18s ease;
+}
+.fade-enter-from,
+.fade-leave-to {
+  opacity: 0;
+}