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/daemon_download_dialog.h

@@ -62,12 +62,14 @@ public:
         }
         using namespace material;
         const float dp = Layout::dpiScale();
+        const auto p = s_updater->getProgress();
+        // No window X during an active download/verify/extract — closing would orphan/block on the
+        // worker; the in-dialog Cancel is the intended way to abort.
+        const bool active = (p.state == St::Downloading || p.state == St::Verifying || p.state == St::Extracting);
         OverlayDialogSpec ov;
-        ov.title = TR("daemon_update_title"); ov.p_open = &s_open;
+        ov.title = TR("daemon_update_title"); ov.p_open = active ? nullptr : &s_open;
         ov.style = OverlayStyle::BlurFloat; ov.cardWidth = 480.0f; ov.cardBottomViewportRatio = 0.94f;
         if (BeginOverlayDialog(ov)) {
-            const auto p = s_updater->getProgress();
-            using St = util::DaemonUpdater::State;
             switch (p.state) {
             case St::Checking:        renderChecking(dp, p); break;
             case St::UpToDate:

src/ui/windows/send_tab.cpp

@@ -889,12 +889,19 @@ static void RenderActionButtons(App* app, float width, float vScale,
     auto& S = schema::UI();
     const auto& state = app->getWalletState();
     double total = s_amount + s_fee;
+    // Block spending from a view-only source (imported viewing key, no spending key) — it would only
+    // fail at submit. Defaults to spendable if the address isn't in the loaded list yet.
+    bool sourceSpendable = true;
+    for (const auto& a : state.addresses) {
+        if (a.address == s_from_address) { sourceSpendable = a.has_spending_key; break; }
+    }
     bool can_send = app->isConnected() &&
                     !state.sync.syncing &&
                     is_valid_address &&
                     s_amount > 0 &&
                     s_from_address[0] != '\0' &&
                     total <= available &&
+                    sourceSpendable &&
                     !s_sending;
 
     float btnGap = Layout::spacingMd();
@@ -934,6 +941,8 @@ static void RenderActionButtons(App* app, float width, float vScale,
             material::Tooltip("%s", TR("send_tooltip_enter_amount"));
         else if (total > available)
             material::Tooltip("%s", TR("send_tooltip_exceeds_balance"));
+        else if (!sourceSpendable)
+            material::Tooltip("%s", "View-only address — no spending key, cannot send");
         else if (s_sending)
             material::Tooltip("%s", TR("send_tooltip_in_progress"));
     }
@@ -1439,9 +1448,13 @@ void RenderSendTab(App* app)
                 ImVec2(colW, memoInputH));
             ImGui::PopItemWidth();
 
+            // The Sapling memo limit is in BYTES (multi-byte UTF-8 fills it faster than visible chars);
+            // warn in colour once the field is at its cap so it's clear input has stopped being accepted.
             size_t memo_len = strlen(s_memo);
-            snprintf(buf, sizeof(buf), "%zu / %d", memo_len, (int)S.drawElement("business", "memo-max-length").size);
-            Type().textColored(TypeStyle::Caption, OnSurfaceDisabled(), buf);
+            size_t memoMax = (size_t)S.drawElement("business", "memo-max-length").size;
+            bool memoAtCap = memo_len + 1 >= memoMax;
+            snprintf(buf, sizeof(buf), "%zu / %zu bytes", memo_len, memoMax);
+            Type().textColored(TypeStyle::Caption, memoAtCap ? Warning() : OnSurfaceDisabled(), buf);
         }
 
         // Divider before action buttons

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

src/ui/windows/xmrig_download_dialog.h

@@ -51,12 +51,14 @@ public:
         }
         using namespace material;
         const float dp = Layout::dpiScale();
+        const auto p = s_updater->getProgress();
+        // No window X during an active download/verify/extract — closing would orphan/block on the
+        // worker; the in-dialog Cancel is the intended way to abort.
+        const bool active = (p.state == St::Downloading || p.state == St::Verifying || p.state == St::Extracting);
         OverlayDialogSpec ov;
-        ov.title = TR("xmrig_update_title"); ov.p_open = &s_open;
+        ov.title = TR("xmrig_update_title"); ov.p_open = active ? nullptr : &s_open;
         ov.style = OverlayStyle::BlurFloat; ov.cardWidth = 480.0f; ov.cardBottomViewportRatio = 0.94f;
         if (BeginOverlayDialog(ov)) {
-            const auto p = s_updater->getProgress();
-            using St = util::XmrigUpdater::State;
             switch (p.state) {
             case St::Checking:        renderChecking(dp, p); break;
             case St::UpToDate: