fix: support libadwaita versions without Adw.Banner

cellar-gtk.py

@@ -156,6 +156,34 @@ def _load_css() -> None:
     )
 
 
+class CompatBanner(Gtk.Revealer):
+    """Fallback banner for libadwaita versions that don't provide `Adw.Banner`."""
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.set_transition_type(Gtk.RevealerTransitionType.SLIDE_DOWN)
+
+        box = Gtk.Box(orientation=Gtk.Orientation.HORIZONTAL, spacing=12)
+        box.set_margin_top(8)
+        box.set_margin_bottom(8)
+        box.set_margin_start(12)
+        box.set_margin_end(12)
+        box.add_css_class("toolbar")
+
+        self._label = Gtk.Label(xalign=0)
+        self._label.set_wrap(True)
+        self._label.set_hexpand(True)
+        box.append(self._label)
+
+        self.set_child(box)
+
+    def set_title(self, title: str) -> None:
+        self._label.set_text(title)
+
+    def set_revealed(self, revealed: bool) -> None:
+        self.set_reveal_child(revealed)
+
+
 # ── Settings Dialog ─────────────────────────────────────────────
 class SettingsWindow(Adw.Window):
     """Preferences dialog using libadwaita widgets."""
@@ -377,7 +405,10 @@ class CellarWindow(Adw.ApplicationWindow):
         # ── Outer box (banner + progress + scrolled content) ──
         outer_box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)
 
-        self.banner = Adw.Banner()
+        if hasattr(Adw, "Banner"):
+            self.banner = Adw.Banner()
+        else:
+            self.banner = CompatBanner()
         self.banner.set_revealed(False)
         outer_box.append(self.banner)