viewer: live detector status bar + dataset-follow sync mode

Add a status-bar cluster, shown when connected over HTTP, that surfaces
the broker state and live acquisition info:
- broker state box (QProgressBar) with progress drawn as the bar fill,
  polled ~1 Hz on its own timer independent of image sync
- Live/Disconnected connection badge (host:port in tooltip)
- "+N new" badge and effective live-rate (Hz) readout
All widgets are fixed-width and only blanked in place, so the bar never
reflows when things appear/disappear.

Add a third autoload mode (HTTPSyncDataset): manually selecting an image
while following live now freezes the displayed image but keeps the dataset,
plots and image count updating. Surfaced via a tri-state HTTP Sync button
(off / live / data-only).

Also fix GetBrokerStatus() dropping the message field and a couple of
copy-pasted exception strings in JFJochHttpReader.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

reader/JFJochHttpReader.cpp

@@ -50,7 +50,7 @@ BrokerStatus JFJochHttpReader::GetBrokerStatus() const {
     auto res = cli_cmd.Get("/status");
     if (!res || res->status != httplib::StatusCode::OK_200)
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid,
-                              "Could not get image buffer status");
+                              "Could not get broker status");
 
     try {
         org::openapitools::server::model::Broker_status input = nlohmann::json::parse(res->body);
@@ -59,6 +59,8 @@ BrokerStatus JFJochHttpReader::GetBrokerStatus() const {
         ret.gpu_count = input.getGpuCount();
         if (input.progressIsSet())
             ret.progress = input.getProgress();
+        if (input.messageIsSet())
+            ret.message = input.getMessage();
 
         if (input.getState() == "Inactive")
             ret.state = JFJochState::Inactive;
@@ -87,7 +89,7 @@ BrokerStatus JFJochHttpReader::GetBrokerStatus() const {
         return ret;
     } catch (std::exception &e) {
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid,
-                              "Could not parse image buffer status");
+                              "Could not parse broker status");
     }
 }
 
@@ -209,6 +211,30 @@ void JFJochHttpReader::ReadURL(const std::string &url) {
         SetStartMessage(UpdateDataset_i());
 }
 
+std::shared_ptr<const JFJochReaderDataset> JFJochHttpReader::RefreshDatasetIfChanged(int64_t &num_images_out) {
+    std::unique_lock ul(http_mutex);
+
+    num_images_out = 0;
+    if (addr.empty())
+        return {};
+
+    auto status = GetImageBufferStatus();
+    num_images_out = status.max_image_number + 1;
+
+    // Re-fetch the dataset (start message + plots) only when the buffer actually changed,
+    // so the dataset-only follow mode does not hammer the broker with plot requests.
+    const bool buffer_changed = !last_image_buffer_counter.has_value()
+        || !status.current_counter.has_value()
+        || last_image_buffer_counter.value() != status.current_counter.value();
+    last_image_buffer_counter = status.current_counter;
+
+    if (!buffer_changed)
+        return {};
+
+    SetStartMessage(UpdateDataset_i());
+    return GetDataset();
+}
+
 bool JFJochHttpReader::LoadImage_i(std::shared_ptr<JFJochReaderDataset> &dataset,
                                    DataMessage &message,
                                    std::vector<uint8_t> &buffer,
@@ -342,7 +368,7 @@ std::vector<float> JFJochHttpReader::GetPlot_i(const std::string &plot_type, flo
             return {};
     } catch (nlohmann::json::parse_error &e) {
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid,
-                              "Could not parse image buffer status");
+                              "Could not parse plot " + plot_type);
     }
 }