fix(migrate): require a reorg-safe sweep depth + re-verify before adopt; make the wallet.dat swap crash-safe

Resilience audit, migrate-to-seed (the sweep RPC is byte-identical — see below):
- Adopt was enabled at >= 1 confirmation and never re-read state before stopping the daemon, so a 1-block
  reorg after the swap could un-mine the sweep and strand the funds in the .bak. Adopt now needs a
  reorg-safe depth (>= 10, matching the History "confirmed" threshold), and the button runs
  verifyThenAdoptSeedWallet(): a read-only gettransaction(confs) + z_gettotalbalance minconf 0 (includes
  unconfirmed, fixing the old minconf-1 "no funds" gate) immediately before the daemon stop, refusing while
  syncing or if any read fails. The remainder re-sweep still shows once mined (>=1).
- The wallet.dat swap was rename(legacy->bak) then non-atomic copy(new->wallet.dat): a hard kill left
  wallet.dat missing or half-written. New order: probe the new file is a BDB wallet; stage a size-verified
  copy beside wallet.dat; COPY (never overwrite) the legacy wallet to the timestamped .bak and verify it;
  then ONE atomic fs::rename of the staged file over wallet.dat. At every instant wallet.dat is the whole
  old or whole new wallet, and a verified .bak exists before the replace. On any failure only our scratch
  files are removed; the user's wallet is untouched.

Byte-identical: z_mergetoaddress (beginSweepToSeedWallet, ["ANY_TADDR","ANY_ZADDR"], amounts, fee, recipient,
memo) and beginAdoptSeedWallet's disconnect/vault-removal/stop/rescan/restart logic are unchanged; only the
adopt gating, two new read-only RPCs, presentation, and the file-move safety change. Pure tests:
sweepConfsAreReorgSafe + adoptReverifyPasses.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -521,6 +521,8 @@ void I18n::loadBuiltinEnglish()
         strings_["mig_sweep_confirm"] = "Send %.8f DRGX from your current wallet to the new seed wallet in a single transaction? This moves all of your funds.";
         strings_["mig_sweep_confirm_yes"] = "Yes, sweep funds";
         strings_["mig_waiting_mine"] = "Waiting for the sweep transaction to be mined…";
+        strings_["mig_waiting_depth"] = "Sweep mined. Waiting for %d confirmations before switching wallets, so a short chain reorganisation cannot undo it…";
+        strings_["mig_adopt_reverify_failed"] = "Could not confirm it is safe to switch wallets yet (the node may still be syncing, the sweep is not deep enough, or funds remain). Nothing was changed - wait and try again.";
         strings_["mig_adopting"] = "Installing your new wallet and rescanning…";
         strings_["mig_done"] = "Migration complete. Your wallet is now backed by your seed phrase.";
         strings_["mig_done_detail"] = "The daemon is rescanning to show your funds — this can take a few minutes. Your previous wallet was saved as a .bak in the data folder.";