fix(ui): prevent stuck spinners and double-submits in dialogs

Address a cluster of low-severity robustness nits where an async
callback could be skipped (leaving a spinner spinning forever) or a
synchronous action could be re-fired mid-flight:

- app_network: add catch(...) fallbacks around the importPrivateKey and
  submitZSendMany worker lambdas so a non-std throw can't escape the
  worker and skip the main-thread callback (stuck "Importing…"/"Sending…").
- export_transactions: re-entrancy guard disabling Export while a write
  is in flight.
- bootstrap_download: disable Cancel once clicked so the request is
  visibly acknowledged and can't be re-fired before the worker stops.
- network_tab: give clear "already in list" feedback on a duplicate
  server (keep the inputs, clear only the stale invalid-URL error) and
  disable/relabel Refresh while a probe is in flight.
- key_export: offer Retry from the error branch (falls back to Reveal)
  and guard against an empty/malformed address before dispatching.
- receive_tab: only enter the "generating…" state when a dispatch is
  actually possible (guards a stuck spinner when disconnected).

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

src/ui/windows/export_transactions_dialog.cpp

@@ -25,6 +25,9 @@ namespace ui {
 static bool s_open = false;
 static char s_filename[256] = "";
 static std::string s_status;
+// Re-entrancy guard: true while a CSV write is in flight (disables the
+// Export button so a second synchronous write can't be kicked off).
+static bool s_exporting = false;
 
 // Helper to escape CSV field
 static std::string escapeCSV(const std::string& field)
@@ -91,14 +94,16 @@ void ExportTransactionsDialog::render(App* app)
         ImGui::Separator();
         ImGui::Spacing();
         
-        // Export button
+        // Export button (disabled while a write is already in flight)
+        ImGui::BeginDisabled(s_exporting);
         if (material::StyledButton(TR("export"), ImVec2(exportBtn.width, 0), S.resolveFont(exportBtn.font))) {
             if (state.transactions.empty()) {
                 Notifications::instance().warning(TR("export_tx_none"));
             } else {
+                s_exporting = true;
                 std::string configDir = util::Platform::getConfigDir();
                 std::string filepath = configDir + "/" + s_filename;
-                
+
                 std::ofstream file(filepath);
                 if (!file.is_open()) {
                     s_status = "Failed to create file";
@@ -138,13 +143,15 @@ void ExportTransactionsDialog::render(App* app)
                     
                     file.close();
                     
-                    s_status = "Exported " + std::to_string(state.transactions.size()) + 
+                    s_status = "Exported " + std::to_string(state.transactions.size()) +
                                " transactions to: " + filepath;
                     Notifications::instance().success(TR("export_tx_success"), 5.0f);
                 }
+                s_exporting = false;
             }
         }
-        
+        ImGui::EndDisabled();
+
         ImGui::SameLine();
         if (material::StyledButton("Close", ImVec2(closeBtn.width, 0), S.resolveFont(closeBtn.font))) {
             s_open = false;