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/daemon/seed_wallet_creator.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <functional>
+#include <string>
+
+namespace dragonx {
+namespace daemon {
+
+struct SeedWalletResult {
+    bool ok = false;
+    std::string seedPhrase;    // SECRET — the caller must wipe it after use
+    std::string destAddress;   // new shielded z-address (the Phase 2 sweep target)
+    std::string tempDatadir;   // datadir holding the new wallet.dat (kept iff keepDatadir + ok)
+    std::string error;
+};
+
+// Mint a fresh BIP39 mnemonic wallet in an ISOLATED throwaway datadir by running a second
+// dragonxd on its own port with no network, export its seed + a new z-address, then stop it.
+//
+// This is the safe first step of "migrate to a seed wallet": it moves no funds and never touches
+// the main daemon or the real wallet.dat. Blocking — call it on a background thread.
+//
+// keepDatadir: keep the temp datadir + its wallet.dat so a later sweep/adopt step can use it, or
+//              delete it immediately (used when only the seed itself is wanted).
+class SeedWalletCreator {
+public:
+    static SeedWalletResult create(bool keepDatadir,
+                                   const std::function<void(const std::string&)>& progress = {});
+};
+
+}  // namespace daemon
+}  // namespace dragonx