refactor(auth): align auth screen with splash screen layout pattern

Extract background image into a reusable AuthBackground component and restructure AuthScreen to use MobileShell as the top-level wrapper, mirroring the SplashScreen implementation.

src/app/auth/components/auth-screen.module.css

@@ -1,22 +1,23 @@
-/* 原始 Dart: lib/ui/auth/auth_screen.dart
- *
- * 视觉规格（与 Dart 对齐）：
- *   - 全屏 `bg_login.png` 背景（cover）
- *   - 500px 内容列由 MobileShell 约束
- *   - 内容列居中
+/* auth-screen.module.css
+ * 与 splash-screen.module.css 同款结构（MobileShell 顶层 + .wrapper + .content）
+ * 删：.fullBleed（mobile 宽度约束改由 MobileShell.content 的 max-width: 500px 提供）
  */
-.fullBleed {
+
+.wrapper {
   position: relative;
-  flex: 1 1 auto;
+  flex: 1;
   display: flex;
   flex-direction: column;
+  width: 100%;
   min-height: 100dvh;
-  background: url("/images/auth/bg-login.png") center / cover no-repeat;
+  overflow: hidden;
 }
 
 .content {
-  flex: 1 1 auto;
+  position: relative;
+  z-index: 2;
   display: flex;
+  flex: 1 1 auto;
   flex-direction: column;
   justify-content: center;
   padding: var(--spacing-lg);