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/app.h

@@ -157,6 +157,8 @@ public:
     config::Settings* settings() { return settings_.get(); }
     // Lite wallet controller (non-null only in lite builds with a linked backend).
     wallet::LiteWalletController* liteWallet() { return lite_wallet_.get(); }
+    // Show the lite send-time unlock modal (called when a spend is attempted on a locked wallet).
+    void requestLiteUnlock() { lite_unlock_prompt_ = true; }
     // (Re)build the lite controller from current settings so a changed lite-server selection
     // takes effect. No-op on non-lite/unlinked builds; preserves a live wallet (see app.cpp).
     void rebuildLiteWallet();
@@ -427,6 +429,8 @@ private:
     bool lite_autoopen_done_ = false;
     // Lite first-run welcome prompt: dismissed for the session once the user picks an action.
     bool lite_firstrun_dismissed_ = false;
+    // Lite send-time unlock: set to show the unlock modal when a spend is attempted while locked.
+    bool lite_unlock_prompt_ = false;
     std::unique_ptr<daemon::DaemonController> daemon_controller_;
     std::unique_ptr<daemon::XmrigManager> xmrig_manager_;
     util::AsyncTaskManager async_tasks_;
@@ -651,6 +655,7 @@ private:
     void renderStatusBar();
     void renderAboutDialog();
     void renderLiteFirstRunPrompt();  // lite-only welcome modal when no wallet exists yet
+    void renderLiteUnlockPrompt();    // lite-only send-time unlock modal
     void renderImportKeyDialog();
     void renderExportKeyDialog();
     void renderBackupDialog();