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.

lintunes/player.py

@@ -3,7 +3,7 @@ from pathlib import Path
 
 from PyQt6.QtCore import QObject, QUrl, pyqtSignal
 from PyQt6.QtMultimedia import (
-    QMediaPlayer, QAudioOutput, QAudioBufferOutput, QMediaDevices)
+    QMediaPlayer, QAudioOutput, QAudioBufferOutput, QMediaDevices, QAudio)
 
 from lintunes.models import Track
 
@@ -53,6 +53,7 @@ class Player(QObject):
         self._counted_finish_id: int | None = None
 
         self._audio = QAudioOutput(self)
+        self._volume = 1.0  # logical 0..1; what the volume slider shows
         self._media = QMediaPlayer(self)
         self._media.setAudioOutput(self._audio)
         # Tee of the decoded PCM, feeding the visualizer
@@ -138,6 +139,25 @@ class Player(QObject):
     def duration_ms(self) -> int:
         return self._media.duration()
 
+    # ---- volume ----
+
+    def set_volume(self, level: float):
+        """Set output volume from a logical 0..1 value (0=silent, 1=full).
+        The slider scale is perceptual, so convert to the linear gain
+        QAudioOutput expects — that makes the knob feel iTunes-like rather
+        than jumping to "loud" in the first few percent."""
+        level = max(0.0, min(1.0, float(level)))
+        self._volume = level
+        self._audio.setVolume(QAudio.convertVolume(
+            level,
+            QAudio.VolumeScale.LogarithmicVolumeScale,
+            QAudio.VolumeScale.LinearVolumeScale,
+        ))
+
+    def volume(self) -> float:
+        """The logical 0..1 volume last set (what the slider shows)."""
+        return self._volume
+
     # ---- transport ----
 
     def toggle_play(self):

lintunes/preferences.py

@@ -10,6 +10,7 @@ DEFAULTS = {
     "highlight": "blue",           # see theme.HIGHLIGHT_COLORS
     "now_playing_font": None,      # None=prefer Century Gothic, ""=use app
                                    # default, "Family"=user-chosen substitute
+    "volume": 0.8,                 # logical 0..1 master output volume
     "lastfm": {
         "api_key": "",
         "api_secret": "",