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/app.cpp

@@ -5081,6 +5081,22 @@ void App::renderSeedMigrationDialog()
         {
             const bool canSweep = !state_.isLocked() && seed_migration_balance_ > 0.0;
             if (!canSweep) seed_migration_confirm_sweep_ = false;   // never keep a stale armed state
+            // The sweep (z_shieldcoinbase / z_mergetoaddress) needs an unlocked wallet, so an encrypted
+            // wallet silently disabled the button. Offer an inline unlock instead of a dead end.
+            if (state_.isLocked()) {
+                static char s_mig_unlock_pass[128] = {0};
+                ImGui::TextWrapped("%s", TR("mig_unlock_prompt"));
+                ImGui::SetNextItemWidth(220 * dp);
+                const bool submit = ImGui::InputText("##mig_unlock", s_mig_unlock_pass, sizeof(s_mig_unlock_pass),
+                                                     ImGuiInputTextFlags_Password | ImGuiInputTextFlags_EnterReturnsTrue);
+                ImGui::SameLine();
+                if ((ui::material::TactileButton(TR("mig_unlock_btn"), ImVec2(120 * dp, 0)) || submit)
+                    && s_mig_unlock_pass[0]) {
+                    unlockWallet(s_mig_unlock_pass, 600);
+                    sodium_memzero(s_mig_unlock_pass, sizeof(s_mig_unlock_pass));   // don't retain the secret
+                }
+                ImGui::Spacing();
+            }
             if (!seed_migration_confirm_sweep_) {
                 if (!canSweep) ImGui::BeginDisabled();
                 if (ui::material::TactileButton(TR("mig_sweep_all"), ImVec2(170 * dp, 0)))

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.";