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/embedded_daemon.h

@@ -191,6 +191,29 @@ public:
     void setZapOnNextStart(bool v) { zap_on_next_start_ = v; }
     bool zapOnNextStart() const { return zap_on_next_start_.load(); }
 
+    /**
+     * @brief One-shot isolated-datadir override for the NEXT start(): run the daemon against a
+     *        different datadir (with its own DRAGONX.conf) plus the given extra args. Used by the
+     *        "migrate to a seed wallet" flow to mint a fresh mnemonic wallet in a throwaway datadir
+     *        without touching the real one. Consumed on the next start(); later starts are normal.
+     *        The caller must serialize this with start() (no concurrent starts).
+     */
+    void setNextStartOverride(const std::string& datadir, std::vector<std::string> extraArgs) {
+        override_datadir_ = datadir;
+        override_extra_args_ = std::move(extraArgs);
+    }
+    void clearNextStartOverride() { override_datadir_.clear(); override_extra_args_.clear(); }
+
+    /**
+     * @brief Skip the "default RPC port already in use → external daemon" bail in start().
+     *        Set true only for an isolated instance running on its OWN (non-default) port
+     *        alongside the main daemon (migrate-to-seed flow).
+     */
+    void setSkipPortCheck(bool v) { skip_port_check_ = v; }
+
+    /** @brief Is an arbitrary TCP port currently in use on localhost? (used to pick a free port) */
+    static bool tcpPortInUse(int port);
+
     /** Get number of consecutive daemon crashes (resets on successful start or manual reset) */
     int getCrashCount() const { return crash_count_.load(); }
     /** Reset crash counter (call on successful connection or manual restart) */
@@ -231,6 +254,9 @@ private:
     std::atomic<int> crash_count_{0};  // consecutive crash counter
     std::atomic<bool> rescan_on_next_start_{false};  // -rescan flag for next start
     std::atomic<bool> zap_on_next_start_{false};     // -zapwallettxes=2 flag for next start
+    std::string override_datadir_;                   // one-shot: -datadir for the next start
+    std::vector<std::string> override_extra_args_;   // one-shot: extra args for the next start
+    bool skip_port_check_ = false;                   // isolated instance on a non-default port
 };
 
 } // namespace daemon