feat(settings): migrate Backup Wallet modal to the reference design

Phase 1. Struct-form BlurFloat overlay (was the legacy positional overload);
DialogWarningHeader care header; centered DialogActionFooter (Create Backup +
Close) replacing the StyledButtons + separator; full i18n — title/destination/
button reuse existing backup_* keys, plus two new keys (backup_warn one-line care
line, backup_overwrite_confirm) added in en + 8 langs. No change to backupWallet.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/lang/zh.json

@@ -62,6 +62,7 @@
     "backup_data": "备份与数据",
     "backup_description": "创建 wallet.dat 文件的备份。此文件包含您所有的私钥和交易历史。请将备份存放在安全的地方。",
     "backup_destination": "备份目标：",
+    "backup_overwrite_confirm": "该位置已存在文件，再次保存将覆盖它。",
     "backup_source": "来源：%s",
     "backup_tip_external": "将备份存储在外部驱动器或云存储中",
     "backup_tip_multiple": "在不同位置创建多个备份",
@@ -70,6 +71,7 @@
     "backup_title": "备份钱包",
     "backup_wallet": "备份钱包...",
     "backup_wallet_not_found": "警告：在预期位置未找到 wallet.dat",
+    "backup_warn": "此文件包含您的所有私钥，请妥善保管。",
     "balance": "余额",
     "balance_history_collecting": "余额历史——正在收集数据…",
     "balance_layout": "余额布局",