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/bootstrap_download_dialog.h

@@ -215,11 +215,15 @@ private:
         ImGui::Spacing();
         ImGui::Spacing();
 
-        // Cancel button
+        // Cancel button — once clicked, disable it so the request is visibly
+        // acknowledged (and can't be re-fired) until the worker actually stops.
         float btnW = 100.0f * dp;
+        ImGui::BeginDisabled(s_cancelling);
         if (TactileButton(TR("cancel"), ImVec2(btnW, 0))) {
+            s_cancelling = true;
             s_bootstrap->cancel();
         }
+        ImGui::EndDisabled();
 
         // Check completion
         if (s_bootstrap->isDone()) {
@@ -234,6 +238,7 @@ private:
                 s_state = State::Failed;
             }
             s_bootstrap.reset();
+            s_cancelling = false;
         }
     }
 
@@ -298,6 +303,7 @@ private:
             s_bootstrap->start(dataDir, url);
         s_state = State::Downloading;
         s_errorMsg.clear();
+        s_cancelling = false;
     }
 
     static inline bool s_open = false;
@@ -305,6 +311,7 @@ private:
     static inline State s_state = State::Confirm;
     static inline std::unique_ptr<util::Bootstrap> s_bootstrap;
     static inline bool s_wasDaemonRunning = false;
+    static inline bool s_cancelling = false;  // cancel requested; disables the Cancel button until the worker stops
     static inline std::string s_errorMsg;
 };