feat(recovery): inline unlock in the migrate-to-seed sweep step

Phase 2 of legacy-wallet migration. The sweep (z_shieldcoinbase / z_mergetoaddress) needs an unlocked
wallet, so an encrypted wallet left the "Sweep all funds" button silently disabled with no way forward.
The Sweep step now, when the wallet is locked, shows a passphrase field + Unlock button that calls
unlockWallet(pass, 600); once it unlocks, the (already-present) canSweep gate re-enables the sweep. The
passphrase buffer is sodium_memzero'd immediately after submit.

No fund-path change: this only unlocks the wallet (the daemon's own walletpassphrase), and the sweep/adopt
logic is untouched. Batched-coinbase progress is already handled — the Confirming step shows the live
remaining balance and the existing "Sweep remaining" flow ("too many inputs for one transaction") drains it
across z_shieldcoinbase's 50-UTXO batches.

Runtime/UI change (build + ctest green; not pure-testable). Still to verify on a real degraded+encrypted
wallet end-to-end: unlock -> coinbase shield -> confirm -> adopt.

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

src/util/i18n.cpp

@@ -512,6 +512,8 @@ void I18n::loadBuiltinEnglish()
         strings_["mig_to"] = "To: ";
         strings_["mig_unlock_first"] = "Unlock your wallet first to spend from it.";
         strings_["mig_sweep_all"] = "Sweep all funds";
+        strings_["mig_unlock_prompt"] = "Your wallet is locked. Enter your passphrase to unlock it, then sweep.";
+        strings_["mig_unlock_btn"] = "Unlock";
         strings_["mig_later"] = "Later";
         strings_["mig_sweeping"] = "Sweeping your funds into the new wallet…";
         strings_["mig_confirming"] = "Waiting for the sweep to confirm on-chain before switching wallets — this guarantees your funds have actually moved.";