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.cpp

@@ -200,6 +200,9 @@ bool Settings::load(const std::string& path)
         }
         loadScalar(j, "wizard_completed", wizard_completed_);
         loadScalar(j, "seed_backup_reminded", seed_backup_reminded_);
+        loadScalar(j, "seed_migration_pending", seed_migration_pending_);
+        loadScalar(j, "seed_migration_dest", seed_migration_dest_);
+        loadScalar(j, "seed_migration_temp_dir", seed_migration_temp_dir_);
         loadScalar(j, "auto_lock_timeout", auto_lock_timeout_);
         loadScalar(j, "unlock_duration", unlock_duration_);
         loadScalar(j, "pin_enabled", pin_enabled_);
@@ -434,6 +437,9 @@ bool Settings::save(const std::string& path)
     }
     j["wizard_completed"] = wizard_completed_;
     j["seed_backup_reminded"] = seed_backup_reminded_;
+    j["seed_migration_pending"] = seed_migration_pending_;
+    j["seed_migration_dest"] = seed_migration_dest_;
+    j["seed_migration_temp_dir"] = seed_migration_temp_dir_;
     j["auto_lock_timeout"] = auto_lock_timeout_;
     j["unlock_duration"] = unlock_duration_;
     j["pin_enabled"] = pin_enabled_;