style(mobile-app): make mnemonic input TextField fully transparent

- Add all border states as InputBorder.none
- Set filled: false to remove background
- Add cursorColor matching theme

🤖 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

@@ -421,7 +421,7 @@ class _ImportMnemonicPageState extends ConsumerState<ImportMnemonicPage> {
               ),
             ),
           ),
-          // 输入框
+          // 输入框 - 完全透明，只保留功能
           Expanded(
             child: TextField(
               controller: _controllers[index],
@@ -431,10 +431,16 @@ 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: 4, vertical: 10),
                 isDense: true,
+                filled: false,
               ),
               textInputAction: index < 11 ? TextInputAction.next : TextInputAction.done,
               autocorrect: false,