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>

src/config/settings.h

@@ -246,6 +246,10 @@ public:
     bool getWizardCompleted() const { return wizard_completed_; }
     void setWizardCompleted(bool v) { wizard_completed_ = v; }
 
+    // Whether the one-time "back up your seed phrase" reminder has already been shown.
+    bool getSeedBackupReminded() const { return seed_backup_reminded_; }
+    void setSeedBackupReminded(bool v) { seed_backup_reminded_ = v; }
+
     // Security — auto-lock timeout (seconds; 0 = disabled)
     int getAutoLockTimeout() const { return auto_lock_timeout_; }
     void setAutoLockTimeout(int seconds) { auto_lock_timeout_ = seconds; }
@@ -444,6 +448,7 @@ private:
     std::set<std::string> favorite_addresses_;
     std::map<std::string, AddressMeta> address_meta_;
     bool wizard_completed_ = false;
+    bool seed_backup_reminded_ = false;
     int auto_lock_timeout_ = 900;   // 15 minutes
     int unlock_duration_ = 600;     // 10 minutes
     bool pin_enabled_ = false;