style(mobile-app): make serial number TextField fully transparent

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/auth/presentation/pages/import_mnemonic_page.dart

@@ -332,9 +332,15 @@ class _ImportMnemonicPageState extends ConsumerState<ImportMnemonicPage> {
               fontFamily: 'Inter',
               color: Color(0xFF5D4037),
             ),
+            cursorColor: const Color(0xFF8B5A2B),
             decoration: const InputDecoration(
               border: InputBorder.none,
+              enabledBorder: InputBorder.none,
+              focusedBorder: InputBorder.none,
+              errorBorder: InputBorder.none,
+              disabledBorder: InputBorder.none,
               contentPadding: EdgeInsets.symmetric(horizontal: 16, vertical: 12),
+              filled: false,
               hintText: '请输入账户序列号',
               hintStyle: TextStyle(
                 fontSize: 16,