ui: QR sharing overlay + Share My QR header on Contacts tab

Adds an lxma://<hash>:<pubkey> QR overlay reachable from Settings and
from a new header row at the top of the Contacts tab. Including the
public key lets Columba/Sideband skip the PENDING_IDENTITY round-trip.

Contacts list stays visible even with no saved contacts so the header
row is always reachable; the empty-state floats above it.

src/ui/screens/LvSettingsScreen.cpp

@@ -957,6 +957,18 @@ void LvSettingsScreen::buildItems() {
         _items.push_back(announceItem);
         idx++;
     }
+    {
+        SettingItem showQrItem;
+        showQrItem.label = "Show QR Code";
+        showQrItem.type = SettingType::ACTION;
+        showQrItem.formatter = [](int) { return String("[Enter]"); };
+        showQrItem.action = [this]() {
+            if (_showQrCb) _showQrCb();
+            else if (_ui) _ui->lvStatusBar().showToast("QR not available");
+        };
+        _items.push_back(showQrItem);
+        idx++;
+    }
     {
         SettingItem initSD;
         initSD.label = "Format SD Card";