Plot tooltip: show cursor timestamp (HH:MM:SS) above trace values

Add a small dimmed timestamp label at the top of PlotTooltip.
set_values() now accepts ts_str as first argument; _on_mouse_move
formats the cursor x-position as HH:MM:SS and passes it through.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui_client.py

@@ -420,13 +420,17 @@ class PlotTooltip(QWidget):
         lay = QVBoxLayout(self)
         lay.setContentsMargins(6, 4, 6, 4)
         lay.setSpacing(1)
+        self._ts_label = QLabel()
+        self._ts_label.setStyleSheet("color: #888899; font-size: 9px;")
+        lay.addWidget(self._ts_label)
         self._labels: list[QLabel] = []
         self.hide()
 
-    def set_values(self, entries: list):
+    def set_values(self, ts_str: str, entries: list):
         if not entries:
             self.hide()
             return
+        self._ts_label.setText(ts_str)
         while len(self._labels) < len(entries):
             lbl = QLabel()
             lbl.setTextFormat(Qt.RichText)
@@ -790,7 +794,8 @@ class PlotTab(QWidget):
             val_str = (f"{y:g} {unit}").strip()
             entries.append((self.COLORS[j], _trace_label(key), val_str))
 
-        tt.set_values(entries)
+        ts_str = datetime.fromtimestamp(x).strftime("%H:%M:%S")
+        tt.set_values(ts_str, entries)
         if entries:
             wpos = pw.mapFromScene(pos)
             tx = int(wpos.x()) + 14