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/daemon/embedded_daemon.cpp

@@ -212,6 +212,11 @@ std::vector<std::string> EmbeddedDaemon::getChainParams()
         "-addnode=node4.dragonx.is",
         "-experimentalfeatures",
         "-developerencryptwallet",
+        // Create fresh wallets from a BIP39 mnemonic so their 24-word phrase can be
+        // exported (z_exportmnemonic) and is portable to SDXLite/ObsidianDragonLite.
+        // The daemon reads this ONLY inside GenerateNewSeed() when a wallet has no seed
+        // yet, so it is inert on existing wallets — safe to pass unconditionally.
+        "-usemnemonic=1",
         dbcache_arg
     };
 }