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/app_network.cpp

@@ -2410,6 +2410,10 @@ void App::importPrivateKey(const std::string& rawKey, std::function<void(bool, c
             else          rpc_->call("importprivkey", {key, "", true}); // label "", rescan
         } catch (const std::exception& e) {
             err = e.what();
+        } catch (...) {
+            // Non-std throw must not escape into the worker — the main-thread callback
+            // below would never run, leaving a stuck "Importing…" spinner.
+            err = "Import failed (unknown error)";
         }
         return [this, shielded, err, callback]() {
             if (!err.empty()) {
@@ -2562,6 +2566,10 @@ void App::submitZSendMany(const std::string& from, const std::string& to, double
             ok = true;
         } catch (const std::exception& e) {
             result_str = e.what();
+        } catch (...) {
+            // Non-std throw must not escape into the worker — the main-thread callback
+            // below would never run, leaving a stuck "Sending…" spinner.
+            result_str = "Send failed (unknown error)";
         }
         return [this, callback, ok, result_str, from, to, amount, fee, memo, markFeeGapRetry]() {
             if (send_submissions_in_flight_ > 0) --send_submissions_in_flight_;