feat(wallet): give new full-node wallets a BIP39 seed phrase + backup UI

New full-node wallets now get a real 24-word recovery phrase, and users can
reveal/back it up from Settings.

- Pass -usemnemonic=1 to dragonxd. The daemon reads it only inside
  GenerateNewSeed() when a wallet has no seed yet, so it is inert on existing
  wallets (safe to pass unconditionally) and makes every freshly-created
  wallet mnemonic-backed — its phrase is then exportable via z_exportmnemonic
  and portable to SDXLite/ObsidianDragonLite. (Existing/legacy wallets are
  unaffected and keep using the chat identity's z_exportkey fallback.)
- App::exportSeedPhrase() wraps z_exportmnemonic on the RPC worker with
  sodium_memzero wiping on every path; it flags the daemon's "not derived
  from a mnemonic" error as a legacy wallet rather than a failure.
- New "Back up seed phrase" modal (Settings -> Backup & Data, full-node
  gated): reveals the phrase in a numbered word grid, copy (45s clipboard
  auto-clear) + save-to-file (0600), wipes the secret on every close path
  incl. dismiss-mid-fetch. Shared ui/windows/seed_display.h grid helper.
- One-time nudge (settings flag seed_backup_reminded) toasts mnemonic-wallet
  users to back up their phrase; legacy wallets are not nagged.
- 15 new strings translated into all 8 languages (additive, 957->972 keys);
  CJK subset font rebuilt for the new glyphs.

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

res/lang/zh.json

@@ -641,6 +641,20 @@
     "sb_warming_up": "正在预热...",
     "search_placeholder": "搜索...",
     "security": "安全",
+    "seed_backup_button": "助记词",
+    "seed_backup_close": "关闭",
+    "seed_backup_copy": "复制",
+    "seed_backup_intro": "这些词是您钱包的主备份。任何拥有它们的人都能控制您的资金。",
+    "seed_backup_load_failed": "无法加载助记词。",
+    "seed_backup_loading": "正在加载您的助记词…",
+    "seed_backup_locked": "解锁您的钱包以显示其助记词。",
+    "seed_backup_none": "此钱包早于助记词功能，因此没有 24 词助记词。请改用“备份钱包”或导出私钥来备份它。",
+    "seed_backup_reminder": "您的钱包有一个 24 词恢复助记词。请立即在“设置 → 节点与安全”中备份它。",
+    "seed_backup_save": "保存到文件…",
+    "seed_backup_save_failed": "无法写入 ",
+    "seed_backup_saved": "已保存到 ",
+    "seed_backup_title": "备份助记词",
+    "seed_backup_warning": "请按顺序将它们抄写下来，离线保存，切勿分享或拍照。一旦丢失，您的资金将无法恢复。",
     "select_address": "选择地址...",
     "select_receiving_address": "选择接收地址...",
     "select_source_address": "选择来源地址...",
@@ -889,6 +903,7 @@
     "tt_save_ztx": "将 z-address 交易历史存储在本地以加快加载速度",
     "tt_scan_themes": "扫描新主题。\\n将主题文件夹放在：\\n%s",
     "tt_scanline": "控制台中的 CRT 扫描线效果",
+    "tt_seed_backup": "显示并备份您钱包的 24 词恢复助记词",
     "tt_set_pin": "设置 4-8 位 PIN 以快速解锁",
     "tt_shield_mining": "将透明挖矿奖励转移到屏蔽地址",
     "tt_simple_bg": "使用简单渐变作为背景\\n快捷键：Ctrl+Up",