feat(wallet): migrate-to-seed, Phase 1 — isolated seed-wallet creation

First, fund-safe step of "migrate a legacy wallet to a seed wallet": mint a
brand-new BIP39 mnemonic wallet in an ISOLATED throwaway datadir so the user
can later sweep funds into it. No funds move and the real wallet.dat / main
daemon are never touched — the isolated node runs concurrently on its own port.

- EmbeddedDaemon: one-shot isolated-datadir override (setNextStartOverride)
  consumed on the next start(); a skip-default-port-check for an isolated
  instance beside the main daemon; and a tcpPortInUse() probe. The datadir's
  basename must be the assetchain name (DRAGONX) and the daemon reads
  <datadir>/DRAGONX.conf itself, so no -conf is passed (both learned from
  live testing against the hd-transparent-keys daemon).
- SeedWalletCreator (src/daemon): picks a free port, writes a throwaway conf,
  starts an isolated dragonxd with -usemnemonic=1 -connect=0, waits for RPC,
  calls z_exportmnemonic + z_getnewaddress (the sweep target), stops it, and
  keeps/scrubs the temp datadir. Off-thread; the seed is wiped after use.
- App: beginCreateSeedWallet (background) -> pumpSeedMigration (main-thread
  handoff) -> renderSeedMigrationDialog (Intro -> Working -> Show seed ->
  Error), reusing the seed_display grid + clipboard auto-clear.
- Settings: "Migrate to seed…" button (full-node gated) + persisted pending
  migration state (dest address + temp datadir) for Phase 2 to adopt.

Validated end-to-end against the hd-transparent-keys daemon: the isolated
node comes up in ~4s and returns a 24-word mnemonic + a shielded z-address.
Requires that daemon (the bundled Jun-28 build lacks z_exportmnemonic). The
migration modal uses English literals for now (as renderBackupDialog does);
the whole flow gets translated once Phase 2 finalizes it. Phase 2 (sweep +
adopt) is next.

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

src/app.h

@@ -9,6 +9,7 @@
 #include <functional>
 #include <thread>
 #include <atomic>
+#include <mutex>
 #include <chrono>
 #include <unordered_map>
 #include <unordered_set>
@@ -352,6 +353,7 @@ public:
     void showExportKeyDialog() { show_export_key_ = true; }
     void showBackupDialog() { show_backup_ = true; }
     void showSeedBackupDialog() { show_seed_backup_ = true; }
+    void showSeedMigrationDialog() { show_seed_migration_ = true; seed_migration_step_ = SeedMigrationStep::Intro; }
     void showAboutDialog() { show_about_ = true; }
     
     // Legacy tab compat — maps int to NavPage
@@ -610,6 +612,10 @@ private:
     // 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();
+
+    // Seed-wallet migration (Phase 1: create a new mnemonic wallet in isolation, no funds moved).
+    void beginCreateSeedWallet();  // starts the isolated create on a background thread
+    void pumpSeedMigration();      // main thread: pick up background progress/result each frame
     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
@@ -678,6 +684,23 @@ private:
     bool seed_backup_no_mnemonic_ = false;
     bool seed_backup_reminder_in_flight_ = false;  // guards the one-time backup nudge probe
 
+    // --- Seed-wallet migration (Phase 1: isolated seed-wallet creation) ---
+    enum class SeedMigrationStep { Intro, Working, ShowSeed, Error };
+    bool show_seed_migration_ = false;
+    SeedMigrationStep seed_migration_step_ = SeedMigrationStep::Intro;
+    bool seed_migration_in_flight_ = false;      // main-thread guard while the bg task runs
+    std::string seed_migration_seed_;            // SECRET — the revealed phrase, wiped on close
+    std::string seed_migration_dest_;            // new shielded z-address (Phase 2 sweep target)
+    std::string seed_migration_temp_dir_;        // temp datadir holding the new wallet (kept)
+    bool seed_migration_backed_up_ = false;      // "I've written it down" confirmation
+    std::string seed_migration_status_;          // main-thread display (progress / error text)
+    // Cross-thread handoff from the background create task (guarded by seed_migration_mutex_).
+    std::mutex seed_migration_mutex_;
+    std::string seed_migration_progress_;        // latest progress line
+    bool seed_migration_done_ = false;           // result ready to consume
+    bool seed_migration_ok_ = false;
+    std::string seed_migration_r_seed_, seed_migration_r_dest_, seed_migration_r_tmp_, seed_migration_r_err_;
+
     // Embedded daemon state
     bool use_embedded_daemon_ = wallet::supportsEmbeddedDaemon(wallet::currentWalletCapabilities());
     std::string daemon_status_;
@@ -940,6 +963,7 @@ private:
     void renderExportKeyDialog();
     void renderBackupDialog();
     void renderSeedBackupDialog();  // full-node "Back up seed phrase" modal (z_exportmnemonic)
+    void renderSeedMigrationDialog();  // "Migrate to a seed wallet" guided modal (Phase 1: create)
     void renderFirstRunWizard();
     void renderLockScreen();
     void renderEncryptWalletDialog();