fix: Tier-2 quick mediums — view-only guard, memo counter, updater/close guards

- Send: block Review from a view-only source (no spending key) with a tooltip,
  instead of failing at submit; label the memo counter as bytes and warn in
  colour once the field hits its cap (Sapling memos are byte-limited).
- Shield: show "No shielded (z) address yet — create one on the Receive tab
  first" when the destination combo is empty, instead of an empty dropdown.
- Updater dialogs (xmrig/daemon): suppress the window X during an active
  download/verify/extract so closing can't orphan/block on the worker — the
  in-dialog Cancel is the way to abort.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/shield_dialog.cpp

@@ -129,7 +129,11 @@ void ShieldDialog::render(App* app)
             }
             ImGui::EndCombo();
         }
-        
+        if (state.z_addresses.empty()) {
+            material::Type().textColored(material::TypeStyle::Caption, material::Warning(),
+                "No shielded (z) address yet — create one on the Receive tab first.");
+        }
+
         ImGui::Spacing();
 
         // Fee