Add volume slider to the transport bar

A slim horizontal master-volume slider in its own slot between the
visualizer and the timeline (TASKS.md). Perceptual->linear gain so it
ramps like iTunes; persists to preferences.json (debounced so it doesn't
thrash the theme). Clicking jumps to the spot via a new ClickJumpSlider
base extracted from SeekSlider.

lintunes/gui/transport.py

@@ -49,6 +49,14 @@ QFrame#nowPlayingBox {
 TITLE_PT_BUMP = 2
 
 
+def _clamp_volume(value) -> float:
+    """Coerce a stored preference to a logical 0..1 volume."""
+    try:
+        return max(0.0, min(1.0, float(value)))
+    except (TypeError, ValueError):
+        return 0.8
+
+
 class FlashButton(QToolButton):
     """Transport button: dark-gray glyph that shows the highlight color
     while pressed (a quick flash on click, sustained while held). A checkable
@@ -101,9 +109,10 @@ def _box(*widgets, margins=(10, 3, 10, 3), hug=False, height=None) -> QFrame:
     return frame
 
 
-class SeekSlider(QSlider):
-    """Timeline slider: clicking anywhere jumps the handle (and playhead)
-    straight to the clicked spot instead of paging; dragging scrubs."""
+class ClickJumpSlider(QSlider):
+    """Horizontal slider where clicking anywhere jumps the handle straight to
+    the clicked spot (instead of paging one step toward it); dragging tracks
+    the cursor. Shared by the timeline and the volume slider."""
 
     def __init__(self, parent=None):
         super().__init__(Qt.Orientation.Horizontal, parent)
@@ -113,7 +122,8 @@ class SeekSlider(QSlider):
             self.minimum(), self.maximum(), int(x), self.width())
 
     def mousePressEvent(self, event):
-        if event.button() == Qt.MouseButton.LeftButton and self.maximum() > 0:
+        if (event.button() == Qt.MouseButton.LeftButton
+                and self.maximum() > self.minimum()):
             self.setSliderDown(True)  # emits sliderPressed
             self.setSliderPosition(self._value_at(event.position().x()))
             event.accept()
@@ -135,6 +145,11 @@ class SeekSlider(QSlider):
         super().mouseReleaseEvent(event)
 
 
+class SeekSlider(ClickJumpSlider):
+    """Timeline slider: clicking anywhere jumps the handle (and playhead)
+    straight to the clicked spot instead of paging; dragging scrubs."""
+
+
 class BpmButton(QPushButton):
     """Tap-tempo button: tap along to the music; when the cursor leaves,
     the shown bpm is saved to the playing track 3 seconds later."""
@@ -231,6 +246,27 @@ class TransportBar(QWidget):
         layout.addWidget(self._visualizer)
         layout.addSpacing(10)
 
+        # Master output volume: a slim horizontal slider in its own slot
+        # between the visualizer and the timeline. The HBox centers it
+        # vertically, so nothing stacks above or below it (iTunes-style).
+        self._volume_slider = ClickJumpSlider()
+        self._volume_slider.setRange(0, 100)
+        self._volume_slider.setFixedWidth(96)
+        self._volume_slider.setFocusPolicy(Qt.FocusPolicy.NoFocus)
+        self._volume_slider.setToolTip("Volume")
+        initial_volume = _clamp_volume(self._prefs.get("volume", 0.8))
+        self._volume_slider.setValue(round(initial_volume * 100))
+        player.set_volume(initial_volume)
+        # Persisting volume re-applies the theme (Preferences.changed), so save
+        # only after the user stops sliding — audio still tracks every tick.
+        self._volume_save_timer = QTimer(self)
+        self._volume_save_timer.setSingleShot(True)
+        self._volume_save_timer.setInterval(400)
+        self._volume_save_timer.timeout.connect(self._save_volume)
+        self._volume_slider.valueChanged.connect(self._on_volume_changed)
+        layout.addWidget(self._volume_slider)
+        layout.addSpacing(10)
+
         center = QVBoxLayout()
         center.setSpacing(0)
 
@@ -302,6 +338,14 @@ class TransportBar(QWidget):
             button.refresh_icons()
         self._apply_now_playing_font()
 
+    def _on_volume_changed(self, value: int):
+        """Slider moved: update audio live, defer the (theme-reapplying) save."""
+        self._player.set_volume(value / 100)
+        self._volume_save_timer.start()
+
+    def _save_volume(self):
+        self._prefs.set("volume", self._volume_slider.value() / 100)
+
     def _apply_now_playing_font(self):
         """Set the now-playing labels to Century Gothic (or the user's chosen
         substitute), black text; title slightly larger than artist/album.