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/app.h

@@ -295,6 +295,13 @@ public:
     
     // Wallet backup
     void backupWallet(const std::string& destination, std::function<void(bool, const std::string&)> callback);
+    // Export the wallet's BIP39 seed phrase (z_exportmnemonic). The callback receives
+    // (ok, noMnemonic, phrase, error): ok+phrase on success; noMnemonic=true when the
+    // wallet's seed is not mnemonic-derived (legacy wallet). Full-node only; the phrase
+    // is a secret and is wiped after the callback returns.
+    void exportSeedPhrase(std::function<void(bool ok, bool noMnemonic,
+                                             const std::string& phrase,
+                                             const std::string& error)> callback);
 
     // Transaction operations
     void sendTransaction(const std::string& from, const std::string& to,
@@ -344,6 +351,7 @@ public:
     void showImportKeyDialog() { show_import_key_ = true; }
     void showExportKeyDialog() { show_export_key_ = true; }
     void showBackupDialog() { show_backup_ = true; }
+    void showSeedBackupDialog() { show_seed_backup_ = true; }
     void showAboutDialog() { show_about_ = true; }
     
     // Legacy tab compat — maps int to NavPage
@@ -599,6 +607,9 @@ private:
     // variants); derives via deriveChatIdentityFromSecret and wipes the secret. No-op when the
     // feature is off, already provisioned, in flight, or unavailable.
     void maybeProvisionChatIdentity();
+    // One-time nudge: on a full-node wallet that has a mnemonic, remind the user (once per
+    // install) to back up their seed phrase. Cheap early-outs keep it idle until it can act.
+    void maybeRemindSeedBackup();
     void provisionChatIdentityFromSecret(std::string secret);
     std::string chatReplyZaddr();                                // a stable (persisted) wallet z-addr for chat
     std::string generateChatLocalId(const char* prefix, int numBytes) const;  // unique echo id / cid
@@ -656,6 +667,16 @@ private:
     bool show_import_key_ = false;
     bool show_export_key_ = false;
     bool show_backup_ = false;
+    bool show_seed_backup_ = false;
+
+    // Seed-phrase backup dialog state. seed_backup_phrase_ holds a SECRET (the revealed
+    // mnemonic) and is wiped with sodium_memzero when the dialog closes.
+    std::string seed_backup_phrase_;
+    std::string seed_backup_status_;
+    bool seed_backup_fetch_started_ = false;
+    bool seed_backup_loading_ = false;
+    bool seed_backup_no_mnemonic_ = false;
+    bool seed_backup_reminder_in_flight_ = false;  // guards the one-time backup nudge probe
 
     // Embedded daemon state
     bool use_embedded_daemon_ = wallet::supportsEmbeddedDaemon(wallet::currentWalletCapabilities());
@@ -918,6 +939,7 @@ private:
     void renderImportKeyDialog();
     void renderExportKeyDialog();
     void renderBackupDialog();
+    void renderSeedBackupDialog();  // full-node "Back up seed phrase" modal (z_exportmnemonic)
     void renderFirstRunWizard();
     void renderLockScreen();
     void renderEncryptWalletDialog();