v0.25.0: New Tracks, a soft emboss, date added pinned

Groundwork for Cassette (friend library sharing) that stands on its own:

- A brand-new library (fresh data dir, or a fresh iTunes import) starts
  with a "New Tracks" smart playlist: added in the last 3 months, newest
  first. Fixed persistent id, so two machines seeding one unsynced share
  merge into a single playlist.
- Every non-flat button, the sidebar Library button and the transport
  boxes get a very slight top-lit gradient computed from the live palette,
  with the chosen color kept exactly at its midpoint.
- Tests pin that individual imports stamp date added = now even for an
  old file, and that the iTunes migration keeps the XML's dates.

lintunes/theme.py

@@ -156,13 +156,56 @@ def apply_theme(app, prefs):
     app.setFont(font)
 
     bar = metrics["scrollbar"]
+    button = palette.color(QPalette.ColorRole.Button)
     app.setStyleSheet(f"""
         QScrollBar:vertical {{ width: {bar}px; }}
         QScrollBar:horizontal {{ height: {bar}px; }}
         QScrollBar::handle {{ min-height: {bar * 2}px; min-width: {bar * 2}px; }}
+        {button_emboss_qss(button)}
     """)
 
 
+def emboss_gradient(color: QColor, pressed: bool = False) -> str:
+    """A barely-there top-lit vertical gradient around ``color``.
+
+    Computed from the live color rather than hard-coded, so it follows the
+    palette (light or dark) and the user's grayscale overrides. ``pressed``
+    flips it, which reads as the button sinking in."""
+    top, bottom = color.lighter(EMBOSS_LIGHTER), color.darker(EMBOSS_DARKER)
+    if pressed:
+        top, bottom = bottom, top
+    # The color itself sits at the midpoint, so what the user picked is still
+    # exactly what the middle of the button shows.
+    return (f"qlineargradient(x1:0, y1:0, x2:0, y2:1, "
+            f"stop:0 {top.name()}, stop:0.5 {color.name()}, "
+            f"stop:1 {bottom.name()})")
+
+
+# How far the emboss strays from the base color (Qt's lighter/darker factor,
+# 100 = unchanged). Kept small on purpose: "very slight, soft".
+EMBOSS_LIGHTER = 106
+EMBOSS_DARKER = 104
+
+
+def button_emboss_qss(button: QColor) -> str:
+    """Global rule for every non-flat QPushButton. Flat buttons (the version
+    button, the cancel-transfer ✕, the bpm label) opt out by being flat."""
+    hover = button.lighter(103)
+    return f"""
+        QPushButton:!flat {{
+            border: 1px solid palette(mid); border-radius: 4px;
+            padding: 3px 12px; min-height: 1.3em;
+            background: {emboss_gradient(button)};
+        }}
+        QPushButton:!flat:hover {{ background: {emboss_gradient(hover)}; }}
+        QPushButton:!flat:pressed, QPushButton:!flat:checked {{
+            background: {emboss_gradient(button, pressed=True)};
+        }}
+        QPushButton:!flat:default {{ border-color: palette(highlight); }}
+        QPushButton:!flat:disabled {{ color: palette(mid); }}
+    """
+
+
 def installed_families() -> set[str]:
     """Families Qt can actually resolve on this machine."""
     return set(QFontDatabase.families())