feat(wallet): give new full-node wallets a BIP39 seed phrase + backup UI

New full-node wallets now get a real 24-word recovery phrase, and users can
reveal/back it up from Settings.

- Pass -usemnemonic=1 to dragonxd. The daemon reads it only inside
  GenerateNewSeed() when a wallet has no seed yet, so it is inert on existing
  wallets (safe to pass unconditionally) and makes every freshly-created
  wallet mnemonic-backed — its phrase is then exportable via z_exportmnemonic
  and portable to SDXLite/ObsidianDragonLite. (Existing/legacy wallets are
  unaffected and keep using the chat identity's z_exportkey fallback.)
- App::exportSeedPhrase() wraps z_exportmnemonic on the RPC worker with
  sodium_memzero wiping on every path; it flags the daemon's "not derived
  from a mnemonic" error as a legacy wallet rather than a failure.
- New "Back up seed phrase" modal (Settings -> Backup & Data, full-node
  gated): reveals the phrase in a numbered word grid, copy (45s clipboard
  auto-clear) + save-to-file (0600), wipes the secret on every close path
  incl. dismiss-mid-fetch. Shared ui/windows/seed_display.h grid helper.
- One-time nudge (settings flag seed_backup_reminded) toasts mnemonic-wallet
  users to back up their phrase; legacy wallets are not nagged.
- 15 new strings translated into all 8 languages (additive, 957->972 keys);
  CJK subset font rebuilt for the new glyphs.

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

src/ui/windows/seed_display.h

@@ -0,0 +1,46 @@
+#pragma once
+
+// Shared helpers for rendering a BIP39 seed phrase as a numbered word grid and for
+// splitting a whitespace-separated phrase into words. Used by the full-node seed-phrase
+// backup dialog and the "migrate to a seed wallet" flow so both present the phrase the
+// same way the lite welcome wizard does.
+
+#include <cstdio>
+#include <string>
+#include <vector>
+
+#include "imgui.h"
+
+namespace dragonx {
+namespace ui {
+
+// Split a whitespace-separated seed phrase into its individual words.
+inline std::vector<std::string> SplitSeedWords(const std::string& phrase)
+{
+    std::vector<std::string> words;
+    std::string cur;
+    for (char c : phrase) {
+        if (c == ' ' || c == '\t' || c == '\n' || c == '\r') {
+            if (!cur.empty()) { words.push_back(cur); cur.clear(); }
+        } else {
+            cur.push_back(c);
+        }
+    }
+    if (!cur.empty()) words.push_back(cur);
+    return words;
+}
+
+// Render the words as a numbered 4-column grid (matches the lite welcome wizard layout).
+inline void RenderSeedWordGrid(const std::vector<std::string>& words, float colSpacingPx = 130.0f)
+{
+    for (size_t i = 0; i < words.size(); ++i) {
+        char cell[96];
+        std::snprintf(cell, sizeof(cell), "%2zu. %s", i + 1, words[i].c_str());
+        ImGui::TextUnformatted(cell);
+        if ((i % 4) != 3 && i + 1 < words.size())
+            ImGui::SameLine(((i % 4) + 1) * colSpacingPx);
+    }
+}
+
+}  // namespace ui
+}  // namespace dragonx