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/pages/settings_page.cpp

@@ -1304,8 +1304,10 @@ void RenderSettingsPage(App* app) {
                 naturalW += ImGui::CalcTextSize(r1[i]).x + btnPadX;
             float wizW = showFullNodeLifecycleActions ? ImGui::CalcTextSize(wizLabel).x + btnPadX : 0.0f;
             float bsW = showFullNodeLifecycleActions ? ImGui::CalcTextSize(bsLabel).x + btnPadX : 0.0f;
+            // Full-node-only "Seed phrase" button trails the Backup button (see below).
+            float seedW = showFullNodeLifecycleActions ? ImGui::CalcTextSize(TR("seed_backup_button")).x + btnPadX : 0.0f;
             float totalW = naturalW + sp * 5;
-            if (showFullNodeLifecycleActions) totalW += wizW + bsW + sp * 2;
+            if (showFullNodeLifecycleActions) totalW += wizW + bsW + seedW + sp * 3;
 
             float scale = (totalW > contentW) ? contentW / totalW : 1.0f;
             if (scale < 1.0f) ImGui::SetWindowFontScale(scale);
@@ -1326,6 +1328,13 @@ void RenderSettingsPage(App* app) {
             if (TactileButton(r1[3], ImVec2(0, 0), btnFont))
                 app->showBackupDialog();
             if (ImGui::IsItemHovered()) material::Tooltip("%s", t1[3]);
+            if (showFullNodeLifecycleActions) {
+                // Back up the wallet's BIP39 seed phrase (full-node mnemonic wallets).
+                ImGui::SameLine(0, scaledSp);
+                if (TactileButton(TR("seed_backup_button"), ImVec2(0, 0), btnFont))
+                    app->showSeedBackupDialog();
+                if (ImGui::IsItemHovered()) material::Tooltip("%s", TR("tt_seed_backup"));
+            }
             ImGui::SameLine(0, scaledSp);
             if (TactileButton(r1[4], ImVec2(0, 0), btnFont))
                 ExportTransactionsDialog::show();