[Client] Graph connection latency history in the status bar

Add LatencyGraphWidget, a size-agnostic bar sparkline over the rolling
sample window: heights scale to the window's own range while colors map
onto an absolute quality ramp, so a steady good ping stays green. Embed
it in the new LatencyStatusWidget together with the textual ping
readout and feed both through ConnectionController's forwarded signals;
the whole area hides while disconnected or without samples.

Took 9 minutes

Took 14 seconds

cockatrice/src/client/latency_status_widget.h

@@ -0,0 +1,38 @@
+/**
+ * @file latency_status_widget.h
+ * @ingroup Client
+ */
+
+#ifndef LATENCY_STATUS_WIDGET_H
+#define LATENCY_STATUS_WIDGET_H
+
+#include <QList>
+#include <QWidget>
+
+class QLabel;
+class LatencyGraphWidget;
+
+/**
+ * @brief Status bar presentation of server round-trip health.
+ *
+ * Combines the textual "Ping" readout with a small LatencyGraphWidget
+ * sparkline of the rolling sample window. Stays hidden while disconnected or
+ * before any samples exist. Owns all latency display state so MainWindow only
+ * needs to forward two signals here.
+ */
+class LatencyStatusWidget : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit LatencyStatusWidget(QWidget *parent = nullptr);
+
+public slots:
+    void updateStats(int lastMs, int medianMs, int p95Ms, int maxMs, int sampleCount);
+    void updateSamples(const QList<int> &samplesMs);
+
+private:
+    QLabel *pingLabel = nullptr;
+    LatencyGraphWidget *latencyGraph = nullptr;
+};
+
+#endif