feat(lite): send-time unlock prompt for locked encrypted wallets

When the user confirms a send on a locked encrypted lite wallet, show an unlock
modal (passphrase -> unlockWallet) instead of letting the backend reject it with
"Wallet is locked". After unlocking, the user re-confirms the send (the form is
preserved). Balances remain viewable while locked; only spending needs unlock.

- send_tab: the Confirm-and-send button routes to App::requestLiteUnlock() when
  getWalletState().isLocked(), else sends as before.
- App::renderLiteUnlockPrompt(): centered modal, passphrase (Enter submits),
  Unlock/Cancel; the passphrase buffer is sodium-zeroed after every path.

Full-node unaffected (gated on liteWallet()/isLocked()). Builds clean, launches
clean, tests pass.

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

src/ui/windows/send_tab.cpp

@@ -771,6 +771,11 @@ void RenderSendConfirmPopup(App* app) {
             Type().text(TypeStyle::Body2, TR("sending"));
         } else {
             if (TactileButton(TR("confirm_and_send"), ImVec2(S.button("tabs.send", "confirm-button").width, std::max(schema::UI().drawElement("tabs.send", "confirm-btn-min-height").size, schema::UI().drawElement("tabs.send", "confirm-btn-base-height").size * popVs)), S.resolveFont(S.button("tabs.send", "confirm-button").font))) {
+              // A locked encrypted lite wallet can't spend; prompt to unlock instead of sending
+              // (the backend would otherwise reject with "Wallet is locked").
+              if (app->liteWallet() && app->getWalletState().isLocked()) {
+                app->requestLiteUnlock();
+              } else {
                 s_sending = true;
                 s_send_start_time = ImGui::GetTime();
                 s_tx_status = std::string(TR("sending")) + "...";
@@ -804,6 +809,7 @@ void RenderSendConfirmPopup(App* app) {
                     }
                 );
                 s_show_confirm = false;
+              }
             }
             ImGui::SameLine();
             if (TactileButton(TR("cancel"), ImVec2(S.button("tabs.send", "cancel-button").width, std::max(schema::UI().drawElement("tabs.send", "confirm-btn-min-height").size, schema::UI().drawElement("tabs.send", "confirm-btn-base-height").size * popVs)), S.resolveFont(S.button("tabs.send", "cancel-button").font))) {