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/config/settings.h

@@ -250,6 +250,15 @@ public:
     bool getSeedBackupReminded() const { return seed_backup_reminded_; }
     void setSeedBackupReminded(bool v) { seed_backup_reminded_ = v; }
 
+    // Pending "migrate to a seed wallet" state (Phase 1 created the wallet; a later sweep/adopt
+    // step consumes it). dest = the new wallet's sweep-target z-address; tempDir = its datadir.
+    bool getSeedMigrationPending() const { return seed_migration_pending_; }
+    void setSeedMigrationPending(bool v) { seed_migration_pending_ = v; }
+    std::string getSeedMigrationDest() const { return seed_migration_dest_; }
+    void setSeedMigrationDest(const std::string& v) { seed_migration_dest_ = v; }
+    std::string getSeedMigrationTempDir() const { return seed_migration_temp_dir_; }
+    void setSeedMigrationTempDir(const std::string& v) { seed_migration_temp_dir_ = v; }
+
     // Security — auto-lock timeout (seconds; 0 = disabled)
     int getAutoLockTimeout() const { return auto_lock_timeout_; }
     void setAutoLockTimeout(int seconds) { auto_lock_timeout_ = seconds; }
@@ -449,6 +458,9 @@ private:
     std::map<std::string, AddressMeta> address_meta_;
     bool wizard_completed_ = false;
     bool seed_backup_reminded_ = false;
+    bool seed_migration_pending_ = false;
+    std::string seed_migration_dest_;
+    std::string seed_migration_temp_dir_;
     int auto_lock_timeout_ = 900;   // 15 minutes
     int unlock_duration_ = 600;     // 10 minutes
     bool pin_enabled_ = false;